(All of this is spoken as a former Aerospray RG main.)
Nintendo has been pretty good at balancing the weapons with patches, so I don't know that anything is really "OP". That said, they tend to balance with ranked in mind, so Turf War ends up being a little bit of an afterthought. The Aerospray is certainly very good at Turf War, although it needs a decent team backing it up to actually win matches.
I've actually had a lot more success using the Aerospray in ranked than in Turf War despite thw fact that it seems to exploit the Turf War ruleset just a bit. But I think that's mostly just my playstyle, I know it's very popular.
I know this is a late reply, but thanks for the info! After making that post, I decided to try out the Jr. again to see what all the hubbub was about, and holy hell I've been living in the dark. I don't know what it is exactly that I like about it so much, but I can see now why people use it even at higher levels.
I've also started doing ranked, which I'm liking a lot so far. Splat Zones and Tower Control are really fun. Rainmaker, not so much.
I used the change in gameplay as an excuse to try out different weapons, and have been rotating them out pretty frequently. I like the Luna Blasters a lot, they're good for getting the other team off of the goal, though I usually end up getting splatted just as many times as I splat others due to their short range. I also tried out sniping with the Kelp Splatterscope, which took a lot of getting used to, but I'm pretty decent at it now. I'm not amazing by any stretch of the word, but I'm no longer dead weight to my team either.
Also for got to put this in my last post; my NNID is Zoriri. Add me if you want a sub-par sniper or a blaster with no sense of personal space.