One thing about drilling your tank and bulkhead size is that the smaller the bulkhead, the more likely you are to create a siphon in your system where the water will be above the bulkhead and will level off above it and therefore reduce surface skimming. By increasing the pipe size behind the bulkhead, IMO, you will not aid your drainage, you will make it vary as you approach creating a siphon, and then lose it as the pipe behind the bulkhead is too large. The larger the bulkhead, the more surface skimming you will get, and the less likely you will be to create a siphon. In very simple terms, the bigger the hole, the easier the water will flow through it. Like the point made above, it will not necessarily change the amount you can handle, only the way in which it is handled. Example: if you drilled a 1 inch hole, the water would come shooting out, whereas a 4 inch hole would be more of a waterfall and the water would exit at less pressure. Another thing to keep in mind is that a siphon might suck your fish up because of the pressure it creates, while a larger bulkhead will allow the water to flow, instead of being pulled, down your overflows. By the way, I haven't seen anything bigger than a 1 1/2 inch bulkhead, but you can use shower floor bulkheads instead (good luck finding a suitable strainer though).
