It will depend upon the piping Code that the juridiction requires you to use, not on the fitting Standard.  For example, if it is a B31.1 Power Piping application:

From ANSI / ASME B31.1 Power Piping Code

124.5 Malleable Iron

Certain types of malleable iron have low ductility characteristics and may be subject to brittle fracture.  Malleable iron may be used for design conditions not to exceed 350 psig or 450 degrees F.

The following reference paragraphs prohibit or restrict the use of malleable iron for certain applicationd or to certain pressure-temperature ratings.

Pipe Supports - 121.7.2(D)
BEP Blowoff - 122.1.4(A.3)
BEP Blowdown - 122.1.4(B.3)
Non-BEP - 122.2(A.1)
Non-BEP Blowdown - 122.2(A.2)
Flammable or Combustible Liquids - 122.7.3(C)
Flammable Gasses - 122.8.1(B)
Toxic Gasses or Liquids - 122.8.2

Best regards, John.

In my "Piping Handbook" by Nayar (excellent book, by the way), it has a table from ANSI/ASME B16.3 that shows the maximum pressure/temperature rating 150# malleable as 150 PSIG at 350*F. There is an asterisk beside the 350*, and a note: "Permissible for service temperature up to 366*F, reflecting the temperature of saturated steam at 150 PSIG."

I take that to mean that, regardless of pressure, 366*F is the limit on 150# malleable fittings, and there's a kind of "special dispensation" that lets you go even that far.

Gentlemen,

OOPS sorry, I missed the point.  Yes class 150 is still class 150.  The P/T rating is what it is and the Code simply adds some further restrictions and limitations.

TBP's report on information from the Piping Handbook is good.  I agree with TBP's "read" on what the Handbook is saying.

Best regards, John.