i'd leave it alone if it were up to me. in the last 3 games his opposing starting SG is only shooting a grand total of 11 for 32...so i'd say he's not killing you as much as you think. maybe you've been giving him some help with DT's but i think that's a very viable strategy if you have one weaker defender amongst your starting 5.
as you acknowledge, part of the reason your SF isn't turning it over is because he doesn't touch the ball...will he be a turnover machine at SG? i don't know about that, but i'm not sure the better defense you're going to get (and with his low speed relative to most guards i'm not even sure how much of an upgrade it will be) is going to be offset by his 0 offensive production