I don't use the trem, so I want to deck it. Can I simply add another spring? Or do I have to adjust the 6 bridge screws after adding another spring?
I'd use a tremble-no before decking the trem, too. It's a good design, works well, and it's easily reversible in case you change your mind or want to resell the guitar.
I've realized that unlike the Strat trem, the PRS trem shouldn't be kept decked. As for the Tremol-no, maybe I'm missing something, but what's the point of spending $70 on it when wood blocks or coins can block the trem (I won't use the trem at all)?
It depends whether you want the resonance that a hollow cavity where the springs and the trem itself are free to resonate. Blocking the trem will absorb some of that vibration and resonance. That absorption isn't necessarily a bad thing, of course. A lot of people prefer it.
FYI, the Tex Mex Strat sells for around $849, not $300.