Last year, I tried a new driller who specialized in milled ovals. He drilled up 3 different size IT slugs for me, and I used them exclusively over a 3-4 month period. They gave a me a great snug feel without having to use more than 1 or 2 pieces of tape. Like you said, when it was there, it was there, but it wasn't always there. Ultimately, I gave up on the ovals. They sometimes created slight hang problems with release changes, and more bothersome,  my thumb swelled and contracted much more than usual. I wasn't used to messing with different inserts during 3-4 game sets, and it became too much of a distraction. I didn't find any magic solution, so I went back to modified round holes with tape. My thumb size stays fairly consistent, and I've been able to use my favorite IT slug 100% the time. I only occasionally add or take out a couple of pieces during long blocks of competition. I have 3 alternate sized slugs that sat unused last year. I haven't looked back since.
 
At higher levels of bowing, thumb tape is not "archaic". Bowl in any PBA tournament, and you'll see most thumb holes layered with black tape and/or white tape tape in the front, back, sides, and every combination you can imagine. It's an inconvenient fact of bowling existence for most better players. It's simply not practical to get the exact feel you need by constantly switching between different sized interchangeable slugs.
 
If your issue with tape is that the Genesis tape doesn't layer well, try using plain old white tape for the base layers. I have the same issue with Real Bowlers Gray tape (RBT), so I put one piece of the gray on top of a few pieces of white tape underneath. The technique works well.
 
My understanding is that the Ultimate Thumb is just another interchangeable thumb system similar to the Switch Grip. I'm not sure how that system would make any difference.

I use tape. 3/4" white. My thumb shape is certainly oval.

I drill a round hole slightly bigger then the biggest sides of my thumb. I then use the white tape in the front and back portions of my thumb hole(the smaller parts of my thumb) to fill the needed gaps.

As a right handed bowler it ends up with tape at 2 and 7 o'clock. I buy a 500 count roll off of ebay every few years and I am good.

I use 2-3 pieces at 2 o'clock and 3-4 pieces at 7 o'clock. When my thumbs swells or shrinks I only add or remove the tape from the back 7 o'clock position. There is no guess work. Every thumb feels identical and is easy to adjust if needed. In most cases I find I need to add or remove one piece of tape from the back.

I just recently went up a thumb hole size from 15/16 to 61/64" for the first time in at least 15 years. Needless to say it feels great and the tape still works amazing for me.

Had the same problems as everyone else. Drove my PSO crazy drilling ITs of various sizes and pitch. Had rounds and ovals. Stumbled on an article in BJI about some pros and the changes they made. Decided my pitch was too much reverse. This is when I drove my PSO nuts. For decades in my area the rule was the more reverse the better, along with a stretched out span. I read that Bill O’Neal has a 1/2” foward pitch in the thumb. That’s unheard of in my area! I’ve gone from 3/8” reverse with tape in the ball and on my thumb to 1/2” foward and 1 maybe 2 on my thumb. Tried to stay with a round hole, of 3 different sizes. Started adding tape to my thumb for that snug feel. After about 3 months my thumb couldn’t take the tugging anymore. Went back to the PS and showed him how to properly use the Ferrari jig. Luckily I had been fitted at the Open Championship by Frankie May and found my oval angle is 35*. Had my thumb hole ovaled by 50 thousandths. Finally had perfect fit. Also shortened my spans 1/8” last 5 weeks of the season I averaged about 220. Snug can turn into too tight in a heartbeat! A few years back I had that happen, popped my thumb at my wrist and at the base of my thumb. Twice in one season! Still paying the price for that. Ovals are awesome but they need to be drilled correctly.

I tried tape-on-tape, and at least using Genesis Orange on the hand and white in the hole, no joy... too much friction.

I also tried RBT, but didn't seem to match up with it. I still have it, maybe I'll try it again....

 
I misunderstood what you previously meant by "tape-on-tape". I thought you meant layering tape in the thumb hole.
 
I can see where Genesis Orange on the back of your thumb and white tape on the back of the hole could be troublesome. There is a lot of friction between those two and it would be easy to encounter hang. I experimented with that combo and I couldn't consistently get out clean.
 
If you just want to stack the Genesis tape on the back of your thumb, try benzoin tincture between pieces of the tape. It's sticky and will help the pieces adhere to each other.

When I use Benzoin Tincture, I only handle it with my left hand (I'm right handed). I use an alcohol wipe to clean any skin reside immediately after. Others play it really safe and use a Q-Tip to apply. You just have to be careful.
 
If you can't use tape in the front of your hole, and white causes issues the back, try using black on the back. I have a teammate who uses nothing but a stack of black on the back, and it works well for him. I can't imagine your Orange Genesis thumb tape sticking against black in the hole, but if there are any issues you can use the slicker Genesis #1 or #2.   
 
Good luck.

Wouldn't all that matters is the last piece of tape that makes contact with the skin?

I will sometimes use white tape in the back because it is thicker than my black tape, then the last piece on top is black for the smoother texture when I need it to be slicker.

Is this what you are talking about lefty?

http://www.ultimatebowling.com/product/the-ultimate-thumb/

Rushing today so I didn't examine completely.  Is the inner sleeve molded to your favorite thumb grip including tape(minus one or two in the front and back)?  I think they used to call that exactacation.

Interesting topic.

Lots of thoughts.

Regards,

Luckylefty