ok well you made a good choice cause im going with intrax as well since i want a nice drop
as for testing, i suppose you can only rip them off your car and test the rebound without the spring mounted - when i installed my shit lowering springs(never buy dropzone) 3 of my 4 struts would slowly "fall" and i would have to pull them back to the top - that was such a bummer hehe
check this out:
http://www.toyotanation.com/forum/t188142.html
thats NOT right
so all i can say is buy GR-2's if that what you want and get intrax or some other good quality spring, as i am doing the same
Mark D.