I believe the wattage limit is the limit of the amount of heat that the lamp holder and fixture as a whole can take, based on Incandescent bulbs. Especially if it is an old fixture.

The 2 watts difference in current is .0167 amps. The ampacity of #18 wire which is the smallest wire I know of in a fixture is 7 amps which which is 840 watts or 20 of those 42 watt loads.

I do go 5% over the limit marked on the fixture myself with CFs but if you are uncomfortable with that, don't do it. I am only telling you what I do and why.

It may be a moot point because the 150 watt CFs might not physically fit.

40 watt equivalent CFs use .075 amps or 9 watts

60 watters use .12 amps or 14 watts

75 watters use .16 amps or 19 watts

100 watters use .19 amps or 22 watts

150 watters use .35 amps or 42 watts

Our bathroom vanities have a long fixture over the mirror with 12 of those dressing room type clear globe incandescent lamps in it. I think they were 40 watts.

I replaced them with 60 watt equivalent CFs and it was so damned bright in there I had to replace them with 40 watt equivalent CFs.

At first my wife said they were ugly but she got used to them.