I've been testing myself every year since 2015. Once a year I restring my racquets to check if I'm serving any slower as I age. Playsight Pro analyzed, using fresh strung 16g VS Touch 1.3mm (59lbs) vs Volkl Cyclone 1.1mm (48lbs) with Pro Red Code Wax 1.2mm (55lbs for gut, 46.5 for VC) crosses on 3 matched swing/twist weight/balance SV98+. I had about avg 2.7% mph overall increase for 1st serves that were in. RPM is supposedly up by 4%, but well within the margin of error so it can't be really accurate.
In real world, the difference isn't drastic but certain serves are better for me. My deuce court wide serves come in 8% more often with gut, because the added power carries the ball over the net. Funnily, I got a bigger increase in speed with the RF97A, a 3.2% mph gain, but I strung it lower too.