To make a quick comment on the "I don't want to invite him because he'll ruin the XP curve," I feel like that mentality mostly applies not to high level characters inviting low levels, but to low level parties inviting high level characters. I think this is what HB is getting at. I love playing Steel. I really love playing him with new characters, new mercenaries, new potential converts to the church of the Dread- er... ahem... anyhow, what most often happens is that I'll bump into a group of low level character in Center, chat with them for a bit, and then when they get ready to go adventure, I find some excuse to wander away, because I know if I go with them, Steel will gimp their progression for the day, and some of them don't get to play that often. You know what else I like to do? Disguise Steel and have him pretend to be someone or something else with characters that don't know him, such as pretending to be some robed, wandering wizard. Works great with new characters, and Steel gets to pretend to be a real mage, since his attack spells can actually hurt stuff in the low and mid-level CR range, heh. Plus, this really limits his power, keeping the whole affair challenging for the party. Except that OOC everyone knows he's way too high level, not only because of his HP, but because once he joined the party, no one got XP anymore, heh. Sometimes the poor sods will humor me for a bit, but deep down, I know they're glad to remove Steel from the party at the first chance.... though, perhaps that's because he's still working on his aim with fireball....
To make a quick comment on the "I don't want to invite him because he'll ruin the XP curve," I feel like that mentality mostly applies not to high level characters inviting low levels, but to low level parties inviting high level characters.
For reference, presently XP starts to be reduced when a player's level differs from the average party level by more than 4 levels. By the time the difference hits 12 levels from the average, XP is reduced to the minimum (which is the creature CR)