You do realize Dynotech used zzp input shafts too right.
The 4340 from zzp is $100 cheaper than intense-racing input shafts. Why don't you compare the strength of the zzp 300m shaft and intense input shaft. The intense shaft is still $25 more than the zzp shaft. I'll admit it does suck that the shaft broke. If ZZP replaced the shaft, I would say thats pretty darn good of them. Most companies won't warranty aftermarket parts for this reason.
Even though intense has their input shafts made by the same company as GM racing does, the shafts are to totally different shafts. The GM Racing shaft looses the lock up feature of the torque converter, which allows them to have a thicker shaft and eleminate the seal groove.