It's not a telephoto, so forgive me if this is off-topic, but I'm often really excited about images from my 40mm pancake lens. It's a huge step up in clarity and sharpness from the lens you have, and it doesn't cost much at all. Also consider picking up a nifty fifty (50mm f/1. to see if wide apertures do anything for you. It feels good to know for sure why you want a particular lens before buying it.
I echo that 100%! I think this rivals some of my Ls in quality - sharp wide open and lovely contrast and colour. The limitations of a prime are really good at forcing you to step up creatively. I think that lens and the 135L make an astounding pairing.
Also, I'd recommend trying the super cheap 70-200 f4 L. The non-IS model was my first L lens and it blew away my consumer 70-300 - nice and sharp and very fast at focusing. It's also light which makes it a lens you'll be happy to take hiking. Best of all you can get them secondhand for £400, which is insane.