"No, it will slow you down as will the sst shifter."
Can't agree. Have you tried driving with shifter long enough, like, at least a month? It's really not that harder and I can't see any reason, why it should be slower. Perhaps in an hour race yes, shifter driver is at a slight disadvantage, since there is some chance of a misshift happening, but as for tiredness, I wouldn't be worrying about that, when you're comfortable with shifter, you don't really use a lot of extra effort to use it.
But overall, in short stints, hotlaps, i'd actually dare to say, that shifter drivers potentially can be faster, cause of, well, fast shifting. But other than this tiny difference, it's still 98% driving skill, feeling the car and stuff like that. There are quite a few WR's done with shifter, Mooney uploaded one not so long ago, french guy Ricou is quite impressive (also has one WR), troy is very fast, and also uses shifter, I've seen Karolis using it online and still showing awesome times.. and I myself got few WRs, mostly LRF stuff.
I'm glad I've started to use shifter, so much fun this way. Can't imagine driving with paddles again, feels the same as it would for regular wheel driver who's trying to drive with mouse.