Questions on my custom 3" catback

[Solid Snake]

I'm almost done building my 3" exhaust system. I'm keeping the stock section of downpipe off the turbo, but I cut off everything starting with the cat. I'm replacing it with a 3" magnaflow high flow cat, a 3" flexpipe and on the rear, a 3" Flowmaster 50-Series big block muffler. I got the muffler's pipe curved over the rear axle properly and I'm using a V-band connector to mate with the downpipe. I'm about 4 welds away from completing it.

I couldn't get the O2 sensor out of the stock cat, not even with a sensor wrench and a hammer. I bought a Bosch 4-wire off Amazon.com and I'm going to splice it in with the stock harness. I'll just plug the bung until it arrives.

I noticed two hooks on the body near the cat. Are these for supporting the exhaust? It seems any rubber hangers used in this area would melt. Should I use springs of some kind? Is welding it to the stock downpipe enough of a support or does the weight of the exhaust cause too much stress on the turbo?

Also, by keeping the stock first 18" or so of downpipe, am I losing much performance or can the hot exhaust gases flow through it fairly easily?

Once I get this stuff sorted, all I have to do is grind down the welds, test for leaks and paint it with VHT