Have fitted the Integy Super-Duty MSR5 shocks (3.5mm shafts) to my Savage X and to be honest, I've not had a problem with them. Maybe I'm just not bashing it hard enough! ( <-- the truck, that is!)
One thing worth noting is that they say to only use 15 or 20 wt oil. I imagine this reduction in resistance puts lets bending stress on the shaft when going from extended to retracted (if not perfectly 'true'), and also reduces the trucks tendency to roll over when cornering at speed. Used with the stock HPI black springs - works for me (so far). I do like the HPI's, however, having spent the money, the Integys will stay until I wreck 'em.