Not really.

Depending on lvl, base and bve status of your engineers/repairers you can get 900 SD at SS4. In that situation, leveling a restorer would be a waste of space, time and thus xp.

Hi,
I would not replace an engineer for a restorer. For the lower tier subs (1, 2 or 3) it could be helpful to have a high lvl restorer to hit the 900 SD. Nevertheless, if you had already level up a BB crew, you could have high lvl engineers and repairers that should give you enouth SD combined with BO and torpers. In my case, I received a lvl 100 over vetted restorer as I began my sub crew, that helped me a lot in the beginning. I do not used it anymore, just engineers and repairers now (I haven't tried the seaman, i think in another threads some sub players have discussed the "seaman on sub" issue). I hope this help you.

Ade 

don't replace the engineer on the sub as that sailor impacts your OH time.

You don't want to shorten your OH time.     

Restorer isn't a must, if one has spent time carefully roll sailors. It might be helpful for low level support, until you can get enough soft defence without him.

As others have said, a restorer is very useful on low level subs. They have very little space and with low DP you would rather have good resistance (900SD) than the abiltiy to repair since most of the time you will be sunk in very little time. 

As you level up and have more space for supports, 900SD can be achieved by a repairer. This is easier if you have leveled a BB before and you have the sailors available.  

From USN SS experience with +11/+12 base crew, the restorer will only help you through SS1-4 alot, once I hit SS5 I didn't need the restorer to hit 900 SD.  Your support slots are VERY valuable since you need both a planesman and a sonarman to function properly, everything else should be engineers so you can keep up with the OH times of BBs.  The only slot your restorer should take (If you choose to, since its a matter of easier lower lvl vs easier higher lvl), should be the gunner slot.

yes.after ss5 u nead for faster stop burning(u are not bb to havexxx slots for 4 repare  4 engine)