I've done that to my AC110 for my 40 gallon tank. But I didn't go all the way down to the bottom just to the middle. Worked out well and kept the water clear for weeks. You should try out an Internal Filters. They works even better!!
internal filters are garbage. useless and bulky IME. at least with HOB and canisters, all you have is the intake in the display. the closest thing to an internal i would ever suggest being used in a tank is a powerhead with a pre filter attachment.and even those can get really bulky sometimes.
Thanks man I just replumbed both of my HOB tonight for under 7 bucks in materials. Works awesome.
Just like ya said for the sand I did another 90 and had a piece that came back up about 6" I didnt drill any holes until about 2" away from the sand. I also drilled my holes with a 1/16" drill bit. Tons of tiny holes so I dont have to clean the java moss out of my impeller every night.