Ive had more than 100 p4s this year (bought and sold them). Ive tried all the speeds and settings. That 2.8 sl6wt wont make it to 3.3 (it will barely do 235 fsb if your lucky).
Of the lot you mentioned, the 2.4 sl6z3 is the best. I also dont know what board you using, but Ive used several. Im using the best right now - MSI Neo2 PLS. All the M0's (which is what the sl6z3 is) run at up to 300fsb and beyond on water and phase change. The best stable overclock I can get on this board with air cooing is 285fsb.
The best available 2.8c out there is also and M0 chip - the sl6z5. They are rare and command a premium.
I have two chips right now a 3.4EE which I may soon sell and a 30cap (M0) 3.2c. Im thinking of going over to a 775 3.4EE when the 3.6EEs come out. 🙂
Sincerely,
FelixDeKat