PCIe gen3 x4 card in PCIe gen2 x8 slot - full performance possible?

Solution 1:

As I understand it, in principle you could do this by putting a PCI express bridge chip* between the slot and the card. Bridges that can do the job certainly exist, it did not take me long to find one on broadcom's website.

In practice though I think such a product would be difficult to get off the ground for three reasons.

1. The overall appeal is pretty niche to start with, you need someone who has an older server, wants to use a newer card, is desperate to eke out the last bit of performance and understands that such a bridge chip can potentially give them that.
2. Physical constraints are a PITA, even if you accept that the card must be low-profile and the host regular profile (which cuts out a bunch of servers just to start with) it's still going to be a pretty tight squeeze to get everything into the space available.
3. Bleeding edge parts tend not to be very accessible to small integrators, most IC vendors tend to make stuff available to their big customers first and the little guys much later if-ever.

* Sometimes also called a switch chip, though that name can be confusing because the term switch is also used to refer to simple signal switches used to re-route PCIe lanes on motherboards.