You can't move it to T3 without making massive changes, like moving the PIV to T4 or something equally as dumb.
It performs fine. It is an RNG machine like all rocket artillery, unless you move it in close (which is the same for Katyusha. It is obviously meant to be used close since it can take 2 shots and still live, and it it is cheaper.
I'm afraid your knowledge is wrong there chap. It has 160 hp. Meaning it dies to one shot, just like katyusha. Its only saving grace is the small amount of armor on it, but that just protects it from small arms. You probably confused it with the Walking stuka (Thats OKW mate)
Source:
http://www.coh2-stats.com/vehicles
Edit: If you did see it take two shots, it was probably in the defensive-bonus area of a command tank; but that's a rare and unusual circumstance.
If it could actually take two shots (like the 251 HT) people would find it much easier to use