The performance of the SU15, are definitely not different from the performance I get from F14 and F15. With my quad-core (but only two CPUs are used by FGFS) and the NVIDIA GeForce GTX 870M with 6 GB of video RAM, the performance remain fairly stable. I turned on the ALS, with 4 covering trees, and advanced weather; all the effects are configured to the maximum value.
Certainly the amount of polygons is high, but well below the 500,000 above which starts detecting a real degradation in performance (-20%), as I could check in earlier tests. It is likely that under-performing graphics cards there may be a slowdown much more noticeable, but still in line with F14 and F15.
This beautiful work of Vitos shows that it is possible to increase the quality of aircraft for Fgfs without losing too much in the quality of the simulation. Instead I have always noticed a big problem for the trees, which are, in my configuration, extremely costly, much more than I expected.