There's not really any busses in modern computers. It's all point to point messaging. You can think of a computer as a distributed system in a way.

PCI has a shared address space which usually includes system memory (memory mapped i/o). There's a second, smaller shared address space dedicated to i/o, mostly used to retain compatability with PC standards developed by the ancients.

But yeah, I'd expect to typically have better throughput and latency with peer to peer communication than peer to system ram to peer. Depending on details, it might not always be better though, distributed systems are complex, and sometimes adding a seperate buffer between peers can help things greatly.