You can actually go a lot over that and still have a functional encounter; in particular, you can fairly safely use brutes that are many levels above the party, seeing as a level +4 brute has the same attack bonus and AC as a same-level soldier. The main problem is that a really high level standard monster, even if it's an XP match for the party, tends to be pretty boring, because standard monsters don't have many abilities to give them flavor. Still, I'm pondering using a Gibbering Mouther (level 10) against a level 3 party.