Actually even for gaming benchmarks only show a 6% performance drop running a card in x4 instead of x16 and it was only like a 3% drop in an x8 slot.
The secret is to just cram as many cards into a computer as possible.
And rather than using a riser/extension card, you can use a soldering iron to melt the end of the slot open so an x16 card will hang in an x8 slot. A lot of server boards don't have ends on their slots for this exact reason. All PCI-e cards are interoperable, it's just a matter of finding a slot to stick them in.
Slot's #2 and #4 are still x8 slots, but since they have no end on them you can plug in an x16 card to them with no riser required. An x1 slot outputs the same power as an x16 slot since power is on the first couple pins of the slot. The only downside is you void the heck out of any warranty by cutting the slots open.