Has anybody installed one of these on there 502 EFI if so what what kind of performance gains did you get.

I am also curious about the (vortex) kit that increases the fuel pressure, spark arrestor/ecu kit to go on a '98 502 MAG MPI.

Any one have any positive/negative feedback? Would the above be more effective with an exhaust manifold/header swap?

I didn't notice any gains with the flame arrestor. I then had the ECU reprogrammed and had the rev limiter bumped up and noticed maybe a mile an hour difference. My next step was to add headers and that did make a pretty big difference. At least 2 or 3 MPH. That still wasn't enough so I added a Vortech blower and gained 12 MPH. Now that I've gotten used to that I'm doing a stroker kit to a 540 with Canfield heads, roller rockers and a custom ground cam. I hope to see another 10+mph when I'm done. You're starting down that addictive road. I wish I would have bought a bigger blue motor in the beginning........
My advice on the arrestor is if you like the way it looks, then buy it but don't expect to spend a few hundred dollars and think your boat will run any faster. The headers will make a difference, plus they look nice too, but they are expensive. I'd say a 1000 dollars give or take equals a mile an hour.

You can gain about 10 Hp with the flame arrester and the ecu. We raise the rev limiter and add a little spark timing to gain the power. The next step is ex and a cam change with a little head work I have the whole upgrade listed on my websight and it will make 510 HP. www.crockettmarineengines.com

But they look cool,UMIN.