On the one hand, there is case for ensuring our MPs receive a salary appropriate to the standing we believe we want to hold them in. Pitching that at mid-level SO1 level seems about right in my opinion.
Not too long ago an MP's salary was tied to that of a Principal in the Civil Service. In today's language that's a UG7/B2. Before the CS pay cuts of recent years it was never far away from this despite the formal link being severed. It was the benefits/pension/expenses etc that made the difference. Until they got caught in the trough a couple of years ago that was the financial incentive they had. I always looked upon them as being of middle officer rank, but with all the benefits and future employment prospects of VSOs.