The SD card interface in your camera can reach up to about 40MB/s write speed.
If your card performs (way) below that speed, SD-UHS won't help.
SD-UHS hack alters the settings of the card writer in your camera, but it only helps if your card is capable of handling those higher writing speeds.
I specially bought a new SD card to make use of the SD-UHS hack. The card that I was using before, maxed out at about 40MB/s writing speed.
And using the highest settings of SD-UHS overclocking, made it even slower, with the overclocking enabled it had max write speed of 21 MB/s.
I'm using a SANDISK EXTREME PRO SD UHS-I (C10, U3, V30) now and it's giving me about 65-70MB/s write speed, that is with SD-UHS overclock option.
As far as I know, the SD-uhs overclocking is reasonable save to use. As long as you click this option,
be sure to wait till it's finished.While the hack is testing/initializing (blinking card reader led) you must wait and do nothing in ML.When card reader led stops blinking, all is set and you're ready to go.
There are also automated LUA scripts out there to enable the SD-UHS hack automatically at startup, but that's the tricky solution that is warned for.
Better enable SD-UHS option manually every time, chances of screwing up your SD card are a lot smaller in that case.
Here's the thread about SD-UHS hack, if you want to know all that is about and risks involved, read all 12 pages:
https://www.magiclantern.fm/forum/index.php?topic=12862.msg185968#msg185968