Combining IP streams requires use of an IP (IETF) standard that's rather complex and wouldn't have much of an audience for WiFi client devices.
Also, it has been said that Wireless is for high mobility devices, or conversely, without mobility, there's no need for wireless (with the exception of too-hard-to-run-cat5-to-living-room). Then, the more mobility one has, the less speed is needed, in broad/general terms.
Stevech - how does 3*3 have anything to do with the IP layer? The streams are combined at the PHY... IETF has nothing to do with the PHY, that's IEEE's job...
And, I'm not certain about your second comment about mobility and speed... MIMO is all about multiple paths and combining, and mobility is an area where MIMO is a benefit with STA's that are truely mobile (e.g. not nomadic - fixed in different locations).
You might be able to reach > 100 Mbps if the distance between the two APs is short and there is little traffic from neighbor networks on the channel you are using for the bridge. You would also need to use 40 MHz wide channels, which you should use only in 5 GHz.
Note that you have a better chance of reaching high total throughput for multiple connections. Single connections will probably not reach > 100 Mbps.