I reckon you should take it back to them (don't tell them how fast you were going) and get them to repair the weld... I've read that for a good aftermarket system they should install a flexi exhaust joint to handle any flexing from the engine. I notice that Landcruisers don't have them but some surfs and Hiluxs had them them...hmm
And regarding taking off the Down Pipe, it is possible, but it is very tight.