Railworks America

what is with the odd looking covers over the exhaust on top of the GP15 and was there a reason for some GP15s to have 4 or even just 2?

Missouri Pacific (later Union Pacific) were buyers of the standard GP15-1 and were the only buyer of the turbo charged version of the GP15. They fitted the “free flow” exhausts to their standard and Turbo locos. All other companies that purchased the GP15-1 had the exhaust as the Loco shown in the photo of the Conrail version. The David Brinkley (DRB) model for TS is of the Missouri Pacific loco so has the “free flow” exhausts which strictly speaking is wrong for all none MP and UP exMP GP15s.
Info from Louis A Marre’s spotters/identification booklets - Kalmbach publications.

The exhaust covers are spark arrestors. They're supposed to capture hot carbon coming out of the exhaust that breaks apart from inside the manifolds once the engine gets hot enough. On my last recording session, hot carbon started a brush fire adjacent to the locomotive, thankfully I was paying attention and quickly put it out.