Personally, I use an engineer, planesman, and sonarman in the support slots, overheat time and speed are 2 of the most important things for playing SS. I also have gunners on the R slots for the flak guns but most people DON'T use them. If you want repairmen on your sub, best option is to put them on the gun slots of your sub since you have 2 and probably arn't planning on using the flak guns. Free support slots should always give priority to engineers since those are the only slots they work in. Repairmen work anywhere.

So long and short, 1 engy in support slot and 2 reps on gun slots

eng+ u flak all the way, but if you have the money to spare, get a elite rep and class as eng (BVE it too), or get an elite eng, and delay classing, if you want high rep rate too+ high SD for the second option (you should cap by lvl 120 anyway), and dont forget to make all your crew annorexic apart from the planseman and eng (but not too thin)

Repairs work 100% effectively no matter where it is.
Engie on Support only.

I'm one of those that use the guns. I've polished off dozens of smoking heaps, sliced squadrons of bombers and fighters with the AA, and vaporized FF/DD/CLs who get too close.

Repair is useless until you're attacked and damaged. If you don't get hit, then repair is irrelevent. If you run out of air near a bunch of enemy battleships, chances are your repairman isn't going to repair you fast enough after receiving a broadside of 16" shells anyway. Repair is when you have enough air to run and hide and lick your wounds after gettting hammered. But, if you are in an SS battle and take damage near your line, then the repairman would be beneficial as you return to the safe side of the BB line.

I've found my L94 Seaman offers more repair than my L80 Repairman anyway....go figure.

i'm 2 lvl's out of the ss4, allways wondered why she only had 3 support slots, but having 2 gunslots would explain it i'd say.

when are you guys hitting 900 SD with at lvl supportcrews on your subs?
i'm used to running a restorer on the gunslot of my SS3 atm, but if i'm planning on using the guns that's not going to be an option anymore.