I did a 136 conversion to my 121 as well. I narrowed a Challenger 136" 2.52 pitch track with 2 inch lugs. I also trimmed off the outside drive lugs, that are just outboard of the guide clips. Saved over a pound of rotating mass. (took an extra 30 min...) Cutting the track was not fun, but good experience. Now I have a good track cutting jig, and the next one should be a lot easier.
I believe my 136 is the same paddle configuration as the 137, just .3 inches shorter lugs. I haven't ridden with the 137 yet, but will Friday.
It went everywhere I pointed it, and was impressively better in the 55+ inches of fresh, over the 121. All we did for the first 1/2 of the day was circle around the sleds to pack it down for them- then pull them out. Rode with a mod 800, a new Pro, and some new Cats. We were the only tracks in Wong's bowl... pretty cool for 55hp bikes. Avy conditions were off the charts- had a 75 yard wide slide cross the road in front of me at 100mph while we were getting a group of sledders turned around to get them out of there. Not cool.
I have ridden the Hawk track (on a few different Hawks) and it's a heavy, stiff track. I'd be interested to know what it weighs- I know Allen has weighed it. I think it's a good option, because it's a bolt in solution. I can see swapping back to the 121 for spring- It shouldn't take more than an hour.
Here are my numbers, from my digital bathroom scale, that weighs in 1/2lb increments:
Timbersled 121- 32.5 pounds
Trimmed Challenger 136 (12.25 wide) 39.5 pounds. It's a 2 ply.
I ordered the same extension kit as mentioned above.
Hopefully this helps-
I should also note- I think a 450 and a 137 would be awesome. The longer track planes out, floats better, and is less work for the bike- especially at less than full throttle.