Okay, here is the opposing viewpoint, LOL.

Lately I have been using a plastic ball for single pin spares and it is 13#.  All of my strike balls are 15#.  I dont really notice any timing issues by going back and forth between the two weights.  

I have noticed that I can throw the 13# harder if I want to, but then my accuracy was off a bit because then I was muscling the shot and that is never good for me.  If I keep from trying to throw harder, my accuracy is just as good as normal.

Earlier in the year I was using a 15# Glowing Amulet for spares but noticed that I would miss the occasional spare because it would hook a bit more than I wanted so I switched over to the White Dot.
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I just want 2C was'zzub.
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I am the SGT Schultz of bowling.
"I know nothing!"

For myself I would get a spare ball around the same weight as my strike ball. You don't want to start really going crazy with increased ball speed at your spares, which I think would happen due to the 3 lb difference in ball weights. Naturally being able to throw 2-3 mph faster at spares is fine for me, but anything more and I know that it would affect my strike game and then I think I would see my average decrease...

Matt