Yes, the professional speed used to be 30ips for the best quality masters, which were typically on 1/2 inch stereo tapes (not 1/4 inch). With the advent of noise reduction systems (Dolby et al), the common speed became 15ips (on 1/4 inch), in order to reduce costs (for the clients). But if no noise reduction was used, it would have been 30ips. The main problem with tapes from those days is that the backing deteriorated within a few years, and if you have tapes that are damaged (I would imagine most of them are, by now), they will likely get stuck in the machine when played. On the other hand, if you can find tapes in new condition, they might sound very good, that's true. Just don't expect to hold on to them for many years.
I'm not sure about the "magic" of cassette tape; it was the worst quality you could find at the time (hiss, narrow frequency response, etc). Recordings made at less than 15ips were often not much better.
Have fun.
