No real way of adjusting the solenoid I can think of.
When I run no-box classes in my altered I have 3 different valves for my brake, 1 steel, 1 aluminum, and 1 composite. Coupled with an adjustable button I can get dialed in on pretty much any tree and rollout. Going from my quickest setup to the slowest is worth .040.
If you know somebody with a lathe you can make a couple of heavy valves and see what works.