Im thinking of getting a FORD RACING 2015-2016 MUSTANG IRS LOADED DIFFERENTIAL 3.55 TRAC-LOK M-4001-88355
and swapping this in place of my Torsen with 3.73's . With my rear wheels it should give me a final ratio of about 3.4.
I could probably sell my stock pumpkin and get most of my money back.
On my 14 with a TVS and a 28 inch tall tire, I was running out of gear 50 feet before the stripe (I think, give or take), revving my sled 400 RPM past my preset 6800 redline. STOCK OPG, mind you.
So, I definitely recommend the 3.31, which is what I have now. I still have a little RPM at the back of 4th crossing the stripe with the same 28.
I don't recommend throwing 5th. The syncrho is usually the culprit under load, even though something else can break.
The 73's are harder off the launch than a 31 as well, so if you want to save a little hurt, it's a good idea as well.
I ran a stock clutch through ~600 lb/ft for 18 months before she took a crap.
I know the diff is a little different in the 15's so my references may be hit and miss, but it really depends on the factors above.