IO540, yes indeed. The 'IR' was once described to me by a CAA IRE as the 'last chance filter to make sure the wrong people didn't get into the airlines'.
Transferring all the non-IR specific knowledge to either the CPL or the relevant TRTO course makes a lot of sense.
But we didn't have to know how many stewardesses are needed in a 747, how to spot illegal lights on moored airships, or the dimensions of the red stripes on the streamers flown beneath tethered kites.....
The IR should be what it used to be in the RAF. A test of the practical and theoretical knowledge requirements appropriate to operating your aircraft in IMC. It varied from type to type, so in the Vulcan we did 45 deg AoB steep turns at FL410 and M0.84, in the Phantom we did 60 deg AoB steep turns at FL300 and M1.2, in the Buccaneer-configured Hunter we did toss bombing profiles at 450 KIAS and 110 deg AoB as part of the profile - and in the Bulldog we did a limited panel no-compass no-gyro SRA approach on the turn and slip after the full panel ILS.
The FAA IR seems far more appropriate for the requirements of operating light aeroplanes in IMC and under IFR than the gold plated JAR-FCL IR.