Using the Taylor Forge method (As in ASME VIII which Caesar uses)I recall calculated bolt loads being too high for 150# flanges when a spiral wound gasket is used. The flange stresses came out OK. This is because this gasket requires a high seating stress.
Although gasket sizes are the same for 150# to 2500# this calculation suggested to me these gaskets should not be used for the 150# rating. B16.5 rating tables make no reservations on gaskets of course. If your equivalent pressure (from pressure + moments) is within the rating then flange is acceptable to B16.5.
Note also Caesar Flange leakage allows you to double (sometimes treble) the bolt allowables. The problem never occurred with 300# which have more and bigger bolts.

Michael