I'm sure you guys know this already but here's a couple of tips for helping with performance issues on a large grid and it doesn't hurt to go over them again....
The biggest performance hit in rfactor is the shadows, if you have them set to full then dropping to high will make a massive difference. The times i have struggled with a full grid, dropping this literally doubled my framerate. best of all - the only thing you lose are the in-car shadows (and possibly from the barriers) but you don't even notice the difference after a few mins.
Doing this i was able to run 32 cars with every other setting maxed out and it was smooth as silk, even at the start. I got the odd stutter passing the pits at Zandy but nothing serious - not sure why this happens here, it's not about frame rate, just seems to jerk a bit for some reason.
Of course if you have already turned down shadows then it probably won't help and setting it anywhere below high makes the game look very odd.
Track detail is also worth looking at - many tracks have a lot of 'hidden' objects which you barely ever see but are still rendered. Turning the track detail down a notch can stop this happening and will increase performance quite a bit. Visually, often you won't even be able to notice the difference.
If you use it, Another thing to check are your AA settings (Anti Aliasing, NOT Alcoholics Anonymous!) Even on modern hardware AA has a pretty big performance hit, if you are stuggling for frame rates then turning it down a notch can help a lot. I've also noticed the in game AA (ie. set through the config app) has much less of a perfomance hit than hardware AA (set through the GFX drivers). So if you have AA 'forced on' through the drivers then change this to let the app have priority and use the rF confing to set AA instead.
No. of visible cars: This has quite a performance hit (especially on DTM where the models seem to be very FPS hungry) however, if you set it to low then you get cars 'popping' in and out of view wich is very distracting. I normally lower the shadows first, then the track detail and then adjust the number of visible cars to as many as i can get away with. A minimum of 10 seems to be a good starting point.
Testing offline is a good performance indicator of what will happen online so start a weekend with 32 opponents and test your performance hit at the start of a race with all 32 cars on the grid and you at the back.
The race start and the first lap is the most stressful for the hardware so you should alter your settings until it's acceptable. It doesn't need to be perfect as things will ease a LOT after the first lap but in needs to be smooth enough that you can get away safely.
You can display your frame race by pressing CTRL+F. Ideally you need to be running above 60+ FPS for most of the race but you can get away with a drop to 50ish at the startline and odd occasions.
On the whole ISI sims are much happier on ATI cards, however some of the more recent drivers (in fact anything since the 10.4's) have caused issues for me - from missing car models to disco's and ALT-Tab lockups. It may not help you but definately something to investigate.
Now i know many of you run rfdynhud without any issues but i've also experienced stutters, runtime errors and lockups when using this. Probably doesn't happen to everyone but worth mentioning.
They are the main things i can think of at the moment. Hopefully you guys can come up with some more