I would just start by bringing all of the maintenance up to current and get the code sorted out. Clean the MAF and check your boost - if you have a gauge and it is hitting target then the turbo should be fine. Otherwise check for boost leaks and test the compression. You could look into an open-source tune from an internet source as well. After that, there is a ton of stuff you can do but a DP and stage II tune would be a nice bump.