Interesting discussion....
I got out of RC back in 2003 and had never really driven a 12th scale, but for half a season on a huge asphalt track running 6-cell Mod. When I got back into things, I started running 17.5 at my local carpet track and have enjoyed it immensely. One of the first things that I noticed was, even with the 17.5s, the cars felt faster to me than the old 6-cell Mod. Track size plays a huge role in the relative feeling of speed with the different turn motors... As does the amount and type of traction.
Since getting back into RC, I have raced 12th at 4 different tracks. My local carpet track, 2 local asphalt tracks(one temp parking lot setup, the other a permanent gas on-road track), and Vegas. 17.5 feels quick enough at my local carpet track. We use the black can Paragon only... and as grip builds, the cars get very edgy, which IMO lends itself to the "relative feeling of speed". At the local parking lot track, it felt slow, but not too bad. The track wasn't too big, but grip was low enough that you had to wait on the cars in the corner... so there was no feeling of speed, neither on the straights nor the turns.
At the larger permanent asphalt track, 17.5 felt terribly slow... that paint-drying feeling. This was with higher grip, too. You could just run pinned, literally everywhere.
When I went to Vegas this year, I ran 17.5 and 13.5. The track layout was similar in size and width to most of the layouts we run locally. One of the first things I noticed, however, was how smooth and easy to drive the cars felt using JTG. Even as the grip got higher, the cars never developed that edgy/twitchy feeling I am used to using Paragon. Coupled with a rather open layout this year, 17.5 felt a bit slower than what I am used to. 13.5 felt really good to me, like a perfect stock speed given the grip level and layout. Having just driven 13.5 the week prior for the first time at my local track, I remember thinking to myself "this would be way too fast for half of our Stock 12th racers"... yet at IIC, it felt very comfortable.
I think it is very difficult to judge what is or is not an appropriate speed for Stock 12th given that there are so many different variables.