4th of July, I flew a new Skyray 35 with a .25LAS, stock muffler, 9-4 (for now, 10-4 after more running). Currently finishing up a Tomahawk II for a Magnum XL .25. It's a much bigger model than the Veco Tomahawk, and the wing is pretty thick. Rich McConnell was flying a Veco Tomahawk with a Fox .15 Schneurle...pretty fast.
The Skyray is on .012's x 60' eye/eye, and I think that's the way to go unless you (or your field) are really hard on the lines. "Line Whip" is ugly, and a small, light model doesn't need it. The .012" lines are plenty strong enough for your Tomahawk. At least, if you don't crimp the ends. Use copper wire and you'll be well served. I cover the bindings with heat shrink tubing, same for leadouts.
My Magnum has some experience on a Pat Johnston "Stunt Trainer" that I flew a lot 4-5 years ago. At that time, AMA rules required .015's, and I regretted that fact. I used an RSM tongue muffler (Fox .35 or OS .40FP/LA stuff all fits). Currently have a RO-Jett tube muffler (35 grams) on it, but will adjust the CG with mufflers. I liked a 9-4 Taipan prop on the "Stunt Trainer" (they're cheap and work well). About $8/10, I think. See Phil Brown's link site for where to get'um or Google "Hobby Club". My venturi is .250" or .257", with a .156" spraybar (K&B, currently).
Steve