U no need guess.... Im one of the um network staff zzzz
Told u is 15mhz is 15mhz...(if u dun believe then i cant do anything also, so no worry)
U get 20mbps doesn't mean u get whole the whole pipe...

Try at 4am to get max speed perhaps all ppl are sleeping.
Get gnettrack. Make sure ur cell id is 1 2 or 3... Then ur SNR must b over 25, RSRP less than -70.
Else i duno how to prove to u that um is using 15mhz....

Upload is always slower as it will interfere with others(im not very sure)...