Bitemark46...going from 4.10 to 4.56 will still affect the dyno...however it may be only 3-4 hp max. Its tough to see that small a diff on a dyno from day to day unless you do alot of runs and avg them out. You say you lost 1-3 mph? The 3mph seems a bit high. There may be other things besides the things we discussed above. Like being a little farther out of you powerband in 4th gear as you go through the traps with the 4.56s. Not sure where your car likes to be shifted. Like I said before...everything is a tradeoff. You could put the 4.10s back in and do more work on your suspension to try and eqaul your 60 foots with the 4.56s.
28" tires may be an option....although changing tire height is the same as changing final gear ratios. 4.56s with 28" tall tires may eqaul out in final drive to 26" tires and 4.10s...however that combo of taller tire with more sidewall/contact patch may give you the launch you need as well as the MPH on the big end. I am really just guessing though...hellifino its a damn mustang!