The difference between the 285's and 255's is only 0.7% (theoretically 6mm in diameter, but it'll depend on the brand). If you are happy with it now, you won't notice the 255's.
Spider gears...as in open diff vs. a spool or always locked? Yeah.
If I was going to regear, I'd do 4.88's. I thought 4.10's & 285's would be worse than they are, but it's not so bad. Last weekend we drove to 10,000ft, then wheel'd to 12,300. I don't have any power complaints...