Transmission:
If used with TOR only the connection is negotiated via TOR, the data itself is still transmitted via insecure channels. So the impact on the TOR network should be minimal and the transfer rate is the same like without TOR.
Don't ask me how that works in detail or if it even actually works at all when the reason for using TOR is to mask your identity.
So technically it shouldn't classify as abuse but morally I think it still does. Let's face it, the #1 reason for using torrent via TOR is illegal activity (e.g. copyrighted material) and everybody who uses TOR for such purposes weakens the legitimation as a means for free information gathering or simply anonymous surfing just because you don't like the big G to know you better than you know yourself.

TOR security:
Basically TOR is a 3-level proxy chain. Your security depends on how trustworthy these proxies in your chain are. There have been rumors in the past of companies or countries running huge TOR server farms. If you by chance get three servers from the same farm, there is no increase in security at all. In fact security even decreases in that case just like it does if you browse via a forced proxy like the one of your mobile internet vendor.
btw: TOR does not implement or replace end-to-end encryption. So don't forget to use ssh just because you're in the TOR network!