So if this was straight up (IE didn't go to ExpressCard) then the performance would be even greater.
Yes. If have a native TB solution then it's 10GBps. The Thunderbolt-to-expresscard adapter limits it to 5Gbps. How does this play out in real life?
13" MBP + GTX660Ti + HD7870 (TH05 @ x2 2.0) tells us:
What performance benefit does x2 2.0 give over x1 2.0?
* external LCD: +4.8%, max=18.1% [GTX660Ti: +6.3% max=18.1%; HD7870=+3.2% max=8.8%]
* internal LCD Optimus: +14.6% max=40.8%
* internal LCD Virtu^$35 : +21.5% max=36.6% [GTX660Ti=23.6% max=36.6%; HD7870=19.5% max=34.2]
Users with IVB/SB expresscard/mPCIe eGPU implementations would likely want to know how much better performance would a Thunderbolt eGPU provide.
We see external LCD sees only +4.8% with max 18.1% performance improvement over x1 2.0. This means the sample benchmarks
are not taxing the pci-e bus. The extra bandwidth showing it's significance when running in internal LCD mode where both Optimus and Virtu benefit significantly from the increased bandwidth.