I think you should be okay
@Aaron MacQ as you aren't putting a smaller diameter on but rather a larger and bringing it down to the stock size if I''m reading your post correctly!! JMO JIM
You're sort of right...i think... maybe....errr...
Basically, as I see it, as the engine draws in air, in reverse order, it flows via the inlet manifold (restrictive), throttle body (64mm), inlet maf pipe (80mm) and airbox (with baffles) and intake snorkel (weird shape, but doubt it's got a greater cross section area than a 70mm diameter, although i'll try and figure that out shortly)
So, engine map aside, more air in should equate to more smiles per miles.
What i'll be doing is using a 70mm sealed cold air intake and free flowing filter to the 80mm inlet pipe, then hitting a 64mm throttle body and finally the restrictive m52 intake manifold.
My understanding of the M52B28 is that the manifold is the most restrictive section of the flow, followed by the throttle body itself. Only, if these were sorted should the 70mm intake become the issue, although possibly it's not a case of 'only as good at the worst part' but more the 'sum of it's weaknesses', and I don't honestly know if the Apollo is any better or worse than a standard airbox.
Then, there's the standard ECU map, which doesn't even try to get the most of the existing breathing by restricting the corresponding fuelling and i think that's why an M50 manifold conversion on it's own results in a loss of torque in the lower revs.
Anyway, we should have the car on the road next week so will run it in for a week or two then test a few combinations.
That's if I haven't found a keenly priced M50B25 manifold in the mean time.
In saying that, someone needs to remind me this isn't even my car.... lol