I don't think you're lookin' at it the wrong way
Just keep in mind that it takes the pumps running full speed to acheive maximum flow
There's nothing wrong with you buying 40s if you're gonna use them for an upgrade later. You would just run them very low speed-wise on your 40b
Keep in mind ...
40s cost a fair bit more and cost lots more for replacement and repair parts
Good thing about them is the wet-side parts are available separately