I actually tried this with my M401 today and was going to start a thread on it (and I still may). The hole in the M401 carts is 1/16" diameter. I simply incresed the size of the hole by 50% with the use of a 3/32" drill bit. The plastic is soft enough that you don't even need a power drill. I was able to do it by hand. I did notice increase in ease of draw as well as a slightly higher vapor production, which in turn gave me a bit of a head rush. I don't think you could get much better than that. Any larger of a hole would be a waste unless you could increase the air intake somehow.
So maybe rather than adding another hole, simply make the current one bigger. Either way, I did notice a difference. Again, this was with the M401. I'm not familiar with the 901.