One accepted way to convert IRC ratings to an implied PHRF handicap is to use the formula:

PHRF = (600/TCC) - 505.

In this case, the .933 TCC for the express implies a PHRF rating of 138. Given the 'real world' PHRF ratings for the boat range from 129 to 141 (I am no Express 27 expert so let me know if I have that range wrong), the implied 138 rating seems to be in line to slightly favorable - i.e. if you race locally with a PHRF rating of 138 or lower, you'd theoretically rather race under IRC all else equal.

Of course, you'd want to convert the handicaps of the other boats in your fleet as well to see how they compare. They might be more or less favored under IRC compared to the Express.

Regarding this .933 number. There is talk that it might be for a masthead spinnaker configuration. This is interesting because we believe that IRC sells spinnaker area cheaply. We also think that foresail area in general and LP's specifically are 'expensive' under IRC. This may be one reason you see a lot of IRC specific designs with fractional non-overlapping jibs and masthead spinnakers with poles around 110% of J. To 'optimize' an Express for IRC I would certainly consider masthead spinnakers, 'penalty' poles, and smaller than class LPs on the genoas as a start.

At least this has been my experience racing under IRC in Asia (but not hugely familiar with the Express 27 per se).