I'm on 10.6.3 with the latest updates for all my pro apps - it's the code base for Logic and Final Cut etc. which needs rewriting to better support hyper-threading and multiple CPUs, you're right.
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I also talked to Adobe about Premier in the CS# versions (seeing if it would be worth shelling out over $1,000), they said they took out some of the optimizing and features; not because of Apple machine capability; but more because they did not want to support 2 versions long term and wanted to concentrate more on Windows users. They said eventually they wanted one code set to maintain, and since the world works on Windows - that was their target.
So see, Adobe admitted to me they were not interested in properly performing Apple products. Let's not blame Apple here (and I am not 100% of a fanboi, I still use some Windows apps); blame the 3rd party software makers for crippling apps.
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When I spoke to Apple support about the terrible performance of the pro apps on my Mac Pro they admitted that, until the code is rewritten, performance isn't going to get any better. It just feels like they've taken their eye off an important subset of Mac / Pro apps users. Anyway, a full evaluation of CS5 on both platforms will make my mind up about whether to go back over to Windows!