I don't think it's so much of an improvement as it is getting rid of a cork. I spend some quality time with some measuring tools a couple weeks ago....
My 3" MAF pipe has a cross section of 6.47 square inches. An LT1 MAF has an area of 4.3 square inches. (66.46% of a 3" MAF pipe)
An LS1 3.5" MAF has a cross section of 5.275 square inches. Even though it's a "3.5"" unit, the actual flow area is only 81.5% of a 3" MAF pipe.
I'm guessing that's the reason for the reports of faster spool. For the longest I always thought it was dumb to put a 3.5"MAF on a 3" pipe as it can only flow as much as the weakest link. Turns out the numbers on the cross section don't lie. Just for kicks a couple weeks ago, I stepped up from my 3"X14" K&N to a LS1 then back down to the 3" pipe. I could her more turbo whistle and there was quicker spool. The butt dyno didn't have an opinion either way.
That being said, if you're blowing the tires off with a 3", a 3.5" isn't going to give any additional performance.