As a test I just merged my two largest routes into a 400+MB monster ("monster" by my standards) and I am seeing a noticeable drop in the FPS, particularly in asset rich (i.e. built up urban) areas. The two separate routes (210MB and 195MB) work perfectly without any issues in SP6.
I always take a minimalist approach to asset selection (e.g. no tree splines or billboard trees, lots of LoRes assets at distances from the track, using assets with LOD data where they were suitable and available, etc) so there is little further pruning that I could do.
I will join you in waiting to see what develops with the next update or TRS26 (whichever is first).