ive thought about this, you really need to set up some sort guide system so you dont remove more than you need to, otherwise youre just increasing case volume and thowing away intake charge / horsepower, your 10mm will end up being an 8mm or a 6mm or something lol
maybe not that much of drastic loss if your careful and take your time.
sit your crank in the case with your rod above the split line and measure how much material is in the way, i think someone on here said some cases will take a 7mm without trenching..
or wait till you got cash and get them machined