FWIW, I don't think downpipe fitment can be considered a factor. There is NO way any one person (Irate, Carson, TN, etc) can build a "universal" turbo mount kit and include a 4" downpipe and guarantee it will match up to every exhaust system out there. If it fits with MBRP, it won't fit with MagnaFlow or Diamone Eye, etc. Every single company does their downpipe different, so expecting the pipes after your existing downpipe to match a 4" downpipe from a different place isn't realistic.
As long as whatever mount kit you use fits along the firewall, and the downpipe doesn't hit the firewall, frame, or transmission, matching up the exhaust after the downpipe will be a necessary part of installing the kit. You might need to modify the first pipe after the downpipe to make it fit, depending on what exhaust kit you have and how it fits. That's pretty minor.