Well, assuming they go mano-a-mano..

Bosch is a tactical Admiral with strategic goals. (He is best at doing things like sneaky evasive maneuvers and defeating larger forces in battle, but it's all part of his Secret Plan.) Thrawn is a strategic Admiral with tactical goals. (He commands from the rear, but his goals are specific - kill Luke Skywalker, destroy this installation, et cetera.)
If you put them in direct confrontation, with each in their own flagship, Bosch has the advantage, being in his element - one on one combat. Thrawn can very well counter some of his moves, but in the end Bosch has the advantage.
The Iceni: Faster, smaller, more powerful anti-cap wise, heavily armored, but with crappy anti-fighter defense. Not equipped with hangar bay.
The ISDII: Slow, large, weaker anti-cap wise (except, maybe, at medium/very close ranges), heavily armored, and with relatively effective anti fighter defense. Hangar bay is loaded with stuff.
That's how it would end up, don't you think? The ISD probably couldn't really hurt the Iceni in a cap versus cap fight, with the Iceni's beams able to hit from long range.. but with hardly any AAAfs or flak, mostly Terran Turrets that could no way hit a TIE and no fighter protection of its own, the Iceni would be battered by TIEs just as badly as the ISD gets smacked by BGreens. It's be a draw.