The way I understand it, he will not be significantly slower. I have heard that the guard skill that is the most affected by height (training time wise) is handling, and even then it's not a MASSIVE difference, but it is noticeable. I think the rest of the skills should be OK. So if you're going to train him as an SG, he should be OK.
And yeah, when you're training an SG, sometimes you'll have to make sure he plays 48 mins in the PG spot (i.e. pressure), even if it's to the short-term detriment of the team. Other types of training, you'll be able to keep him at the SG spot.