Lighting – Remodel Recessed Lighting in New Basement

I personally would abandon them for something with better requirements - there's a reason that IKEA stuff is low in price, and that reason is compromises. For instance, the GORM shelving units are strong and flexible, but take a while to assemble and are kinda ugly.

In this case it looks like the compromise is the required spacing. My guess is that the heat load they produce (especially the temperature of the fixture itself) is high enough that they need the extra space to cool (a different, more expensive, fixture might have some other way to ditch the heat that would require less space).

That said, any engineer worth his salt (and his lawyer) isn't going to skimp on the required spacing. It's very likely that you could get away with shaving a few inches off of it, but there are no guarantees if you start violating the spacing requirements.

The biggest worry in my mind would be that if something went wrong (even if it had nothing to do with these fixtures), the insurance company would balk because these fixtures were installed 'improperly'.

The fixtures you linked to are absolutely fine for new construction sheetrock ceilings. If you are going to use suspended acoustical tile then you will need a different model fixture that mounts directly to the tiles, not the floor joists.

In either case, you can lamp them with CFL bulbs which will burn cooler, less electricity. The only disadvantage to standard CFL's is that they can not be dimmed with an inexpensive incandescent dimmer. Always be sure to match the dimmer and bulbs correctly.