FS: GT28RS - Single or Pair

Yes

Yes. In both applications you will have to do some minor trimming of the heat shields if you intend on using the stock manifolds.

A set of higher flowing fuel injectors are always recommended when increasing the volume of air entering the intake tract. Less fuel usually means more chances for detonation. If your fuel pump is up to the task, then injectors should be upgraded.

Yes it is possible but not recommended. You should always at least upgrade your fuel system by increasing the size of the injectors. A band-aid solution would be to increase the fuel pressure by means of adjustable FPR, but it is not a consistent and reliable change to keep you out of the detonation happy zones.