What are you unimpressed with? Sharpness, contrast? Bokeh?
Heavy tripod doesn't mean much at all; tripod coupling can be just as important as the actual weight of the tripod. Did you use MLU? Always use MLU on the RB if you can. Even on a heavy tripod, if your coupling is a bit dodgy, you can get some ringing that will not damp out quickly enough. Just now somebody will come on and say MLU doesn't matter, and thankfully I won't see it because he's on my ignore list, for good reasons. Use MLU. Be happy.
I have the 90 KL and the 127 KL; I prefer the 127. I got the 90 just because I needed that exact field of view. I'd say the 127 KL is a tiny bit better all 'round, but just a bit. Likewise I have the 180 and the 210 KL apo and prefer the latter. But I am not one to do side by side tests, I simply use what I like.
Oh and yes always use a shade, esp. if you are using pre KL lenses. Just now somebody will jump in with some idiotic comment about modern coatings not mattering, and again I won't see it because he's on my ignore list, but mark my words, use a freaking shade and then we'll talk If you don't shade, you are simply giving away contrast. Anyway, granted, the rb lens hoods aren't chic, they look like toilet plungers. But they are inexpensive and they collapse nicely. Get one for each lens and be happy.
So: good tripod coupling. MLU. Shade. Happiness. If not, return the lens, and pick up a 127 KL. Even more happiness.