Fundamentally and legally, you only have a MG if you get more than one round firing when you pull the trigger one time. After that, we are venturing into the land of policy, opinion, and the vast legal gray area that belongs to whoever has the money to either fight there or is the USG...in which case money is no object b/c the USG has an unlimited supply of money and lawyers thanks to your tax dollars.
Leaving aside who will actually do a modification, it seems clear the SP5 holds some good promise as a sear host provided:
1. It has a FULL SIZE SHELF indistinguishable from that on the 10,000s of legal HK-style guns out there (and NOT a smaller shelf like a Zenith). If the SP5 doesn't have a "standard" or full sized shelf you would need to permanently add one.
2. It has NO FRONT PUSH PIN HOLE the same as the 10,000s of legal HK-style guns out there (and NOT a more "cosmetically correct" hole like a Zenith). SP5 achieves the paddle mag release without a through-and-through hole like 1,000s of other HK-style guns out there now.
With both of these features in place, someone stupid enough to seek an illegal MG conversion will need to both defeat the shelf and the lack of a pin hole, i.e. a very deliberate action demonstrating clear ill intent (and not insignificant metal-working skills -- you can do it with a Dremel tool, but you have to want to do it).
The receiver block is reminiscent of the old "no F/A carriers" in the AR-15 platform in the sense it removes metal from the area it would interface with a F/A sear (or "catch" in HK lingo). Time and practice have removed the sensitivity of a "standard" carrier in an AR, and I believe the same practices and principles apply to the HK -- specifically running an unmodified carrier in either an AR or an HK is not a significant step in an illegal conversion as a standard carrier will only contribute to F/A fire if first you have an auto-sear (and proper F/A hammer)...and all focus should properly be on these parts rather than the rather benign carrier.
So if modified carriers are so insignificant why does the HK (and other imports) have them? Because the ATF is the gatekeeper on importing these pistols and has decided that as a matter of POLICY they will demand them for access to the U.S. market. It may not be a great thing for U.S. consumers (added cost, non-standard parts, etc.) but it is great thing for a USG bureaucrat to say they have gone "above and beyond" the requirements of the law and can never be reproached for their work by their superiors. U.S. consumers have no representative in the ATF approval process to protect their interests, and until the law changes you are gonna find increasingly funky modifications to imported guns. The good news is that there is still a measurable amount of freedom available to us once a gun is here in the U.S., and responsible firearm owners can modify their firearms consistent with the law.