In converting the bugbear I used a leader bugbear from the Monstrous Manual and the method described in the DM Options: High Level Campaigns to add +1 toughness. So he looked like:
Bugbear Leader (AC 3, Mv 9, HD 4+4, hp 27, Thaco 15, #AT 3/2 footman's flail, Dmg 1d6+5, SA -3 surprise penalty for opponents, SD nil, MR nil, Morale 13)I figured he would be fairly tough.
He was dead in 3 rounds. He did manage to knock Gabriel the swashbuckler unconscious in the process however. I can tell already though that if I want a dangerous and dramatic fight I either have to make single monsters tougher or increase the number of monsters in the converted encounters.