It depends on the ski and also the water you run in...
As a general rule, the more aggressive grate GRABS more water which creates more drag. So, it can actually slow the ski down.
If you ride in ruff water the grate will normally help as your grate is often not normally fully submerged. It will throw water into the impeller better than a stock grate as it extends further down and has a more extreme angle.
If you have a super charged engine or other items that improve the engine power, you can use this grate with either minimal loss all the way through to improved performance.
With a stock engine, you will get nice out of the hole acceleration and normally get improved cornering as well. If you have a STRONG engine, and especially if you have an improved impeller, you may get as much as 3 mph on the top end as well...
I have bought but not installed just yet some goodies myself for my RXT-X. Bought Solar Impeller, Wedge, Steering Extender and performance grate. Looking forward to summer....