This advice is just based on your concern about the speed.
Although the instructional on Incredible Pi mentions that the eMMC on BBB is much faster than the uSD, this is not the case on other OS to uSD ports for example on Cyanogenmod for Android OS to microSD.
I am also running an Android OS from a microSD card on Nook HD+, and the card
random write speeds make all the difference in the overall speed and experience when running OS from external memory card.
It is tested by myself and others in Cyanogenmod dev that Sandisk(and some Transcend) SD/uSD are vastly faster than cards from Kingston, Samsung, and others in respect to random write speeds. In fact, some report random write speeds on some Sandisk Class 4 SD as faster than eMMC on Nook HD+.
Personally testing between cards that have 0.20- to 1.0+ random write speeds but similar sequential speeds, I can tell you that it makes a HUGE difference in OS performance - OSes do not write sequentially often, and the random write speed is the bottleneck when running OS from SD card.
So, whenever using SD/uSD for running OS, I always choose Sandisk(I use UHS-1/Class 10 instead of the recommended Class 4 because the 6 or so cards I have tested have high enough random write speeds in addition to fast sequential speeds for imaging, copying, etc.) and test the speed for 1.0+Mb/s random write.
"Random Write 4KB (QD=32)" is the category to watch in Crystaldiskmark.
I am currently running three Incredible BBB on Sandisk 16GB Class10/UHS1 uSD cards with this typical specs from Crystaldiskmark - these were taken from a USB3.0 external adapter - the random write speeds are similar when using USB2.0:
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CrystalDiskMark 3.0.3 (C) 2007-2013 hiyohiyo
Crystal Dew World :
http://crystalmark.info/
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* MB/s = 1,000,000 byte/s [SATA/300 = 300,000,000 byte/s]
Sequential Read : 36.854 MB/s
Sequential Write : 13.481 MB/s
Random Read 512KB : 35.817 MB/s
Random Write 512KB : 2.366 MB/s
Random Read 4KB (QD=1) : 5.454 MB/s [ 1331.6 IOPS]
Random Write 4KB (QD=1) : 1.396 MB/s [ 340.7 IOPS]
Random Read 4KB (QD=32) : 5.699 MB/s [ 1391.3 IOPS]
Random Write 4KB (QD=32) : 1.204 MB/s [ 293.9 IOPS]
Test : 50 MB [F: 0.0% (0.0/14.8 GB)] (x5)
Date : 2014/06/02 10:36:36
OS : Windows 7 Professional SP1 [6.1 Build 7601] (x86)
I will expand root partition on BBB and test the eMMC when I have some time - taking some time off soon, so may be awhile.