Is this doable in a long day?
Matthew and I did it last Sunday. Whether or not it's enjoyable in a long day is another (subjective) matter :)
We didn't see a soul after the upper lake. There looked to be some nifty places to camp as well. Base-camping up there and then scrambling the nearby peaks over a few days could be a lot of fun. Sawtooth or Caesar Cap might be interesting - Matthew said Wayne Moss' book on the Trinity Alps had some beta on Sawtooth, but I don't have that one (yet).
Just a thought - It's 8 miles to the upper lake. Then about 2 or 3 miles of cross-country travel (boulder-hopping and scrambling included). Maybe 20-odd miles round-trip, with ~6000' elevation gain. The real fun starts above the upper lake, since it's about 3000' of gain from there.
The Hobo gulch "long way" is not mentioned on this page. While long, it is an incredibly beautiful hike through big trees, alpine forests, meadows, Trinity Alps history, and then to the high alpine. It can also be combined with a trip up to Papoose, or up to Thurston Peaks for an amazing multi-day wilderness experience.
"This route has a 8000 feet elevation gain. About 10 miles."
This should read 6000 feet, not 8000 feet