Bandwidth:
"Good enough" is entirely subjective, and depends on what you wish to use it for. However, for a more objective view, we can compare the bandwidth. Wikipedia is as good a resource as any for this purpose.
PCIe 2.x x16: 8 GB/s
[External ports are also possible with an] ExpressCard interface, which provides single lane v1.1 performance.
The page appears to be a bit outdated, since ExpressCard 2.0 provides about double that - close to single lane PCIe 2.x. See below.
ExpressCard 1.2: 2.5 Gbit/s (PCI Express mode)
ExpressCard 2.0: 5 Gbit/s
PCIe 2.x x16 is 8 GByte/s, which is about 64 Gbit/s, which is over 10 times the bandwidth of ExpressCard 2.0.
The majority of high end video cards recommend PCIe 2.x x16. Therefore, the bandwidth will be lower then they require to run at full capacity. Does this affect performance? Yes. Does it appreciably affect performance? Depends what you want to do. I don't think newer 3D games will run well.
Latency
I couldn't find any direct information on this. However, I will say that latency is almost guaranteed to be higher - you have an additional cable, compared to a slot sitting on the motherboard. Whether this will affect performance at all (let alone in any appreciable way), I have no idea.