4, 7 and 10 mil are common.
For the price it costs to have a custom 5 mil done, or other odd number...it won't offset the small performance gain if any...IMO.
However, if you got a line on a cheap 5 mil stroker crank, the other additions/mods would be the same as a 4, 7 or 10 mil crank in perspective. The head and porting would have to be matched.
The big difference is this, bigger than 4 mil, you will need to trench the cases most likely. 4 mil is a drop in crank, no case work.
Once you go bigger...say 5 or 6, you MAY or MAY NOT have to trench.... 7 mil and bigger you will, hands down. Figure that into your costs. Trenching runs 75 to 150 bucks.