Definitely do another one and TIG it. Also, what filler wire are you using for the stainless to stainless... and what about the stainless to mild? You should be using 308 for the stainless and 309 for the stainless to mild. Got any pics of the inside so we can see penetration?

Also, you shouldn't be welding this stuff without cleaning it first. I use a stainless wire brush on a bench grinder and Acetone right before welding... ofcoarse, I TIG mine. You should just buy a TIG welder and learn instead of taking it to a weld shop, where they'll charge you an arm and a leg to weld that up. You can get by with a fairly small welder, I usually only use 70-90 AMPs for sch10 stainless piping.

Pauls right, do not powdercoat it... that will melt. Either ceramic coat it or polish it.

Did you test fit this manifold with your turbo yet? I bet clearance in very tight, if there is any at all.

Nice first effort... manifolds can be a PITA sometimes.