the darkest beer i've done w/ them is oktoberfest where the imported malt is absolutely essential. i'm not much of a hef guy (don't dig the banana/clove), but when I was looking for my lagers to be 'creamier' and I couldn't put my finger how to describe it... weyermann base malts did the trick. Assuming there aren't any huge yeast/hop/roast ect. flavors in the way, german malts taste nearly like a luxurious dessert to me, whereas american base-malts just taste like regular beer. Ive had the best results w/ lighter german styles mashing short and hot, 156-157 for 25 minutes.
I've always assumed the yeasty character of a hefeweizen covered up any base malt subtlety, but this thread may show that that line of thought was wrong! if you switch it up for a batch, please post results