Considerably beneficial, from what I recall.
It’s been quite awhile since I ran the swapped Silvia Gr.4, and I think I only ran it once (maybe twice?), so I can’t remember the specifics in regards to how many laps you can get at what FM setting or what the weather was when I did, but I do remember it was actually quite a good increase in overall getting around the track-ability and was considerably faster with the swap. I don’t remember having to pit after every single lap like some road some cars require, but even if you had to, especially in the races where you get the rain and need to pit for different tires anyways, you’re still going to come out “better” with the swap. I think I got 2 maybe 3 laps per tank with a high FM setting, but not sure what FM I was set on or what the weather impact was on pitting. It was a much easier win than with the stock Gr.4 though. But don’t expect to build such a huge lead you can sit at the line for 5 mins after lap 6 to end the race right at 30 min. like you can with other swapped cars.
Having said all that, I think there have been at least 3 or 4 updates since I did that and at least 1 of them has had a significant impact on the PP valuations because of physics changes, so it might be a very different experience in 1.34.