If Pierce goes, I think KG will not be back in Boston either. KG strikes me as the type of player who would follow Pierce at whatever price it took to spite the team that traded the guy he's battled with for five years.
I think the more likely scenario is Bradley and our two 1st round picks for #7 and #30. Basically you're giving Bradley and the 21st for #7. I'd probably do that trade if I were Portland. Bradley has proven himself as one of the best defenders in the league and that he has a developing offensive game as well. There is no guarantee with the 7th pick as to what you'll get...
For me, I'd rather keep Bradley and see how far I could move up with my two first round picks, maybe to #14 or #15.