Locke,
Please realize that the Mercury and the Mars used different cores. Yes, it was only turned upside down, but the differences it produced in ball reaction was much more than that.
The Mercury version created balls that were much more rolly in nature, whereas the modified core created and more "normal" reaction: more length (not a lot) and more backend. I am almost shocked that your Mars was less ball than the Mercury, but I wonder if some of that is not attributable to its rollier nature (it tried to set up earlier than the Mars).
If you need something stronger than the Mercury (I read this as earlier and handling more oil. Please correct me if I'm wrong), given the lack of Planet series balls available, I'd suggest a Riot (the solid one) with a good amount of polish and a longish pin-PAP distance, maybe in the range of 5" (maybe 5.5"). The Riot normally handles up to medium oil, but polish + the longish pin should drop it down to handling just a little more than the Mercury.
"None are so blind as those who will not see." "Some bowlers are crazy as pickled cats."