FLying some of the more complicated IFR approaches is pretty damn difficult, in VMC or IMC - it makes no difference.
Your point is a good one, but I think you exaggerate. Flying IFR in VMC can indeed be "pretty damn difficult" in some circumstance, but handflying a light aircraft in IMC undoubtedly adds to the workload, otherwise we wouldn't bother with those pesky screens and hoods for practice. IMC also takes away the safety net of seeing the obstacle before you hit it.
Where there is a big difference for a relatively inexperienced private pilot is that, unless practising, he will be flying an approach for real, with few of the toys that Sick Squid has at his disposal, and the pressure will be on. He may be disorientated, worried and distracted by even a small equipment failure. Add in night and poor weather, slight traces of icing... We owe it to him to ensure he is trained to do it. If his training is less than that of an IR, then to higher minima.
I think one of the points IO540 is making is that to force the pilot to fly at low level and deprive him of an air traffic control service because the airspace above him has the wrong letter attached is hardly going to make his job any easier. Yet the difficulty of flying IFR for IMC-rated pilots tends to be increased by the airspace class restrictions. To fly IFR with an IR is usually considerably easier than what they have to deal with, as we have more levels available and more ATS help. That is yet another reason why a reduced-privilege instrument qualification must not be arbitrarily restricted by airspace class.
And I have reservations about any restriction being based on IAP minima. A couple of hundred feet vertical buffer sounds as if it's a concession to inexperience, but is unlikely to save the poor overloaded pilot who turned the wrong way at the fix, misread the platform altitude from the plate by 1000 ft, selected the wrong VOR (these days I guess it's "flew towards ABAMO rather than ABUMO"), or misheard the ATC clearance and failed to check the MSA. The vertical tolerance with which one flies a profile has a lot more to do with currency and experience than the amount of training one received 10 years ago at an approved organisation.