Well the WHR-HP-G54 certainly isn't MIMO - it's "MIMO Performance*" - note the "*":
*This product uses a two antenna/single high power transmitter technology. It is not designed to anticipated 802.11n standards. Based on Buffalo Technology outdoor tests, this product outperforms two radio/three antenna MIMO technology beyond certain distances. Buffalo Technology testing also shows that this product equals or exceeds the performance of two radio/two antenna MIMO technology at all distances.
but it's still a very good router. There are alternative firmwares available to give more functionality/flexibility. DD-WRT is supported through the
WHR-HP-G54DD which provides a myriad of options. Currently, I'm using one called
Tomato
Enjoy.