I play a duskblade ruled exactly this way and I believe it's not unbalanced at all, just a bit more interesting.
Things like keen weapons or the improved critical feat makes critical feel al lot less exciting: but when rolling a 20 means 10d6 instead of 5d6 you put things back in the right...
I was skeptical myself about the first question, since even if a large greatsword could be about as long as a guisarme you'd still be trying to hit with the middle of the blade, and not with the tip.
The second though was a bit unexpected: it seemed strange to me that I would get a global...
I'm playing a duskblade who recently decided to go around with a large greatsword (obviously thanks to monkey grip). Even if it could be considered a waste of a feat (I already had power attack, which would've made the best use of the -2 penalty) it added the flavour I was looking for, and I was...