Ah, the fin! I like the fin, but I see no reason why you can't just leave it off, as long as your opponent knows what you've got it upgraded with (if anything). I will say with nothing on the back, I'm not sure if you'll like it, but you'll see what I mean.
Anyway on performance, I guess I could follow up to my own post... they've been solid. What a fun little unit. So far I've taken the armour save upgrade only. It just seems to take all the Heavy Bolter type kills out of the game.
On weaponry, that's another matter! I was sure I would go with and stick to gauss. I am second guessing that now. I love the gauss but I find my 'need' or want for the double tap at 12" caused me to get a little too close, too often.
Also I'm not sure on the math on this but it certainly felt like the overwatch situations were stronger with the Tesla ability. So now I like the Tesla slightly more, but it takes tank hunting out of the question for the most part.