I bought a set of the 944 +55mm Jonbloed wheels from Phils tire for my 87 944 chassis. I installed the same Toyo RR 225/50/15 tire. Car was spec legal except for wheels when I tested this tire and wheel package.
The car was raced at Willow Springs in California at a Nasa event two weeks ago. The current lap record is 1:35.9 My fastest lap that weekend was a 1:34.2
This was not a particularly fast weekend as most classes were nowhere near the lap records for their class.

The wheel change appears to be good for a minimum of 1.7 second reduction in lap time for $800 plus shipping.

The wheel weight difference was -2.5 lbs per wheel.

The stack height difference of 4 mounted tires between the 15x7 stock wheel and the 15x9 wheel with was 4.5".

I am in no way proposing going to a wider wheel for spec944. But if you run additional classes like I do such as Gts1 or PTD this certainly gives you a more competitive car for minimal dollars.

I will be testing the new 225/45/15 RR tire at Buttonwillow next month. Surprisingly the 45 series tire is not wider than the 50. It does help with acceleration because it is shorter.

Very cool information Charlie- thank you for sharing!

Update, we ran the 944 in the enduro at Buttonwillow 2 weeks ago. We ran the 45 series tire and the performance was lacking. Just barely faster than the Spec track record. The entry and mid corner speed is sub par.
I switched to the 50 series tire and the car was 4 seconds faster. The corner speed is so much higher.
The only change was the tire. Both sets are using the same wheel.
I raced the car in PTD at 2740 lbs due to car prep rules and points. I had to add weight due to internal engine mods.
I qualified on the pole and won the race with 7 cars in class.
Was DQ for being light by 10 lbs at weigh in. Car burs alot more fuel in sprint races as compared to enduro shift points.

So you stretched a 225 on to a 9" rim and went faster? That's pretty good!
I would have thought the 8" rim to be optimal for that size.

There was no streching going on. The rim is not wider than the tire.
Now with the 225/45/15 the rim is wider than the tire. You wouldnt think that would the case but it is. The 45 series tire on the 9" rim is just slightly faster than the spec setup.

Some of the Miata PT guys are putting a 205/50/15 Hoosier on a 9" wheel and going alot faster. Not sure you could pull that off with the RR.

OK so help me out here. I too will be running a different series so I
don't have to use the stock wheel. I have an 85.5 which is the early
offset of +15?? I'd like to get the 9" rim for all 4 corners. Which
offset do I get? The +35 or +55?? I want to keep it square. Is that
possible?

Thanks,
Ken