Yeah, I wouldn't go more than a couple pounds over the stock level. After that, you're just putting extra unnecessary strain on the turbo. The T-25 is so small that it ends up running out of breath and you will not see any gain from boosting it higher.

If anything, you'll lose performance, because of the turbo spiking and then dropping boost. If you want to push extra boost, ditch the T-25 and get at least a 14b or bigger.