I think it will depend on the HP of the engine being tested. I saw no gains in the boat switching from Gils to CMI's on my hopped up 525SC. Something with more HP would probably see more gains.
Stainless Marine Gen III's make more power on blower motors than most. The ports are huge and the flow really well. They also have a 5 yr warranty.
I've had both CMI and Stainless on my boats and was not happy with the CMI's and went back to Stainless. On the boats I've worked on the Gils last a decent amount of time but were prone to cracking near the exhaust pipe flange. I heard they've since fixed that problem. On the boats I've worked on the Stellings held up pretty well and nothing but problems with CMI's over the years.