Approx how many miles do you ride in a typical year? Many people on here change from the stock IRC and claim improved performance. You can spend the $$$ on Metz if you ride high miles, where you "might" wear out the tire before dry rot sets in...or you can save some $ on another brand if you ride less.
I swapped my stock IRC for a set of Shinko 712 and I couldn't be happier
For more options, you can swap the stock rear for the 140/90-15 (I did too) but you might have to swap a couple of bolts that hold the saddlebag brackets to mushroom head bolts to avoid tearing up your new tires.
Whatever you decide, remember to take it easy the first 100 miles or so, those tires will be slick at first.