In all honesty, if you expect to run this sim maxed out on the kind of resolutions iMac monitors boast you're only setting yourself up for frustration
What kind of settings are you running?
I'm running mostly medium settings on a two year old PC (i7 920 @ 2.7Ghz, Ati 4890 1GB and just 3GB of RAM, using win7 x64) and i actually increased my trees amount with this patch, plus it runs like butter with minor stutters (not constant ones, just loading stutters when textures/effects are loaded for the first time, then it's ok). The only case i get real stuttering is if i'm skimming roof tops low over London (less than 500ft) and even then, it mostly hovers around 20 FPS or so (it's the only situation i did use an FPS counter, since i thought it might be useful to know what is my minimum FPS for my settings to make a decision on whether i should adjust them further).
In this patch FPS might be a bit lower (i don't use FPS counters so i can't be sure, plus i don't use the benchmark black death track because it's recorded from an earlier version and this can cause problems too, at least it did in the previous IL2 series), but it seems much more consistent and smooth now.
What i'm trying to say is, i'd rather have a steady 40 FPS than 60 FPS that fluctuates down to the mid 20s at times. It's the same reason many demanding flight sims have been using FPS limiters in the past (FSX is a notable example), if the sim knows it will need to render less frames for a given time it's easier to keep ahead of the curve and render them in time, instead of giving you 50 FPS when looking over sea and then dropping to 20 FPS when you look back towards land.
I can't really know if 5FPS is something that should push you over the edge because i don't know your starting average FPS. If you're getting 40-50 FPS then it's a trivial matter, if you're getting 25-30 FPS then losing 5 FPS can be an issue.
I also don't know what kind of settings you are running but i'll go out on a limb here and suggest the easiest performance fix available.
The best advice i can offer you is to lower your texture size in the graphics options. It's the setting with the highest impact (along with SSAO) in terms of FPS and to be honest with you, dropping it less than two notches results in minimal, almost imperceptible quality loss. Original size textures are just that, the original, unscaled, unoptimized, raw textures, they use up tons of VRAM.
Try this and tell me if it helps, just drop your texture size a notch and you might not have to lower anything else.