In general I've found:
L+1-2: Standard - bloodied and or drop multiple, low chance for some death, almost no chance of TPK
L+2-3: Boss - some death(s)
L+4-5: Endgame, a TPK or near TPK is fine
Optimized or have a few key ingredients (warlord, radiant mafia): +1 monster worth
Glory (have notable optimization, for example their strikers can reliably kill 1-2 monsters in round 1, each): +3 monsters worth
Insane (never lose init, can kill an entire encounter, then follow up with another): +6 monsters worth
Wrong (plays a different type of D&D, most likely using rules loophole that a DM shouldn't allow for things like infinite damage, etc): +10 monsters worth
So generally you just need to up one thing to an elite or one elite to a solo, not the whole encounter.
If you have a big group (more than 4 or 5), you might need to bump things even more, of course.