To my knowledge, the only person who's tracked a Goblin with a non-stock frame behind the rollbar makes over 600whp, runs 275 wide 100TW tires, has a lot of track experience, and is running the prototype DF subframe and rear suspension, so no, you are unlikely to find the rear frame section somewhat flexible if you plan to track your car. Everyone else with Goblin track time is running a stock rear frame and subframe and is much faster on track than an unfinished Goblin.
The harness bar makes sense to me. Some folks have added tabs to help with mounting the side panels. No other gussets or reinforcements required. In my opinion, anything else is a project you choose to do for fun and stands between you and a running driving Goblin.
The only downsides to powdercoating are the challenge of touching up and cost. Its strength is near-unmatchable.