I have used PC-TEA and there is no problem getting adequate development without sulfite. I think I used a slightly incorrect amount of ascorbic acid so my times for FP4+ at 1:50 at 68 degrees F were 13 minutes or longer. Whether some combinations give actual superadditivity or just seem that way is an academic question. Once you have the time/temperature/EI figures worked out, you are in business. I have read that hydroquinone has either very little activity or no activity below a certain temperature. That temperature might be about 60 degrees F. When the hydroquinone does get active I don't know whether its activity increases in a liner or non-linear way as temperature increases.

Even if digital photography had not advanced as quickly as it did, there would still have been pressure to reduce the toxicity of the chemicals used. Color processing was mostly done on a larger scale and for many years the spent chemicals were picked up and treated in some way rather than just being dumped somewhere in an unsafe way. There isn't that much large scale traditional b&w processing being done now. This makes it even more important for small scale users to learn how to handle the chemistry safely. I think there were three factors in the interest in phenidone over metol in recent years. The first was that phenidone is less toxic than metol. Secondly, even though successful developers combined phenidone with hydroquinone, ascorbic acid is far less toxic than hydroquinone. If you use PC-TEA you even eliminate the sulfite and the amount of TEA used in a 1:50 dilution is quite low. Finally, the newer technology films and the improved old technology films (like Tri-X) respond better to phenidone than the older films did. The increase in grain is not very apparent and the improvement in sharpness is welcome.

You can do a lot with PC-TEA and do it more safely than with Rodinal or pyro type developers. I think that if developing times were too long or if we were using large rubber tanks with replenishment then superadditivity would be more relevant in practical terms. I any case this thread has been interesting to read.