The only time I reboot either Mac at our house is when the software updates require it. I even had an Ubuntu instance running in Virtualbox and it hasn't slowed my Mac down one bit.
I usually reboot every couple weeks. Its a good thing to clear out all the memory and start clean, but its rare that I ever have any performance issues. As I said, this machine is a beast. 8GB of ram definitely helps.
How are you zooming in Safari, with gestures?
How much ram does your machine have?
Don't have Final Cut Pro, so can't help there, but I don't have any issues with Safari, don't use it much though. I use Chrome for a browser, with Safari as my backup.
I am thinking there is a setting you need to tweak for your mail settings, but hard to say. I like the Safari Mail.app, but actually use Sparrow.app It is a wonderful mail program. Very fast, supports gmail almost natively - complete with labels, and works well with my work IMAP settings.
Right now I have 4gb... upgrading to 8 tomorrow but haven't had the time to do so.
With the internet if I use firefox / chrome / etc there are no problems. Using safari with gestures brings up the rainbow circle almost religiously. today has been exceptionally bad. I took it to the store and it tested fine so it appears to be software based.