You are absolutely correct about the importance of sprue plate holes. There is no one-size-fits-all.
If the sprue hole is too big for a particular bullet/alloy, it may be difficult to cut the sprue, it may result in a bumpy base, and it may push the base out of round.
If the sprue hole is too small for a particular bullet/alloy, it may be difficult to achieve good fill.
The other side of the coin is that it takes time and money to maintain an inventory of sprue plates in a variety of sizes. That's why most manufacturers offer only one size. Inventory, or lack thereof, has been a huge problem for Mountain Molds, and it's one reason why we have been forced to eliminate some mold options.
Mountain Molds currently offers two sizes of sprue holes for each block. The most appropriate of the two will be shipped with your mold. At best, the sprue size will be suitable only for the specified alloy -- if you change alloys, all bets are off.
From time to time I am asked about making a custom sprue hole size. Since the sprue hole size is determined prior to countersinking, heat treating, grinding, and bluing, a custom size means a custom, one-of-a-kind sprue plate. Estimated shop time 8 hours (probably longer, but I'm trying to appear reasonable). Multiply 8 hours times a fair shop rate. That's in addition
to the standard cost of a mold.
Sorry I could not tell you what you wanted to hear.