yes, thats a known problem with the p51/50.cals.
Wags said that this is a problem with the current engine, and will only be fixed with EDGE, so hopefully soon.
just like you i have the same impression, that your own tracers dont cause that much of a performance hit, but watching other p51s firing their guns has a big impact.
interesintly though, i do not have the problem with the fw190...or at least not as obviously as with the p51s.