Basically no or very minimal loss of spice is observed with FASA. Any losses are purely "mechanical" as you wrote. And since one may pull with xylene/toluene till the plant material is exhausted, the total yield will be quite higher than the other methods.
The "jungle spice" story is quite obscure. People have been extracting it with xylene or toluene evaporating it down and washing it with solvents to remove dmt. People have used different solvents, different methods and have gotten different end products. There are no standardised methods so as to aid the comparability of the various jungle spice reports.
Now, FASA can be used to salt a whole range of alkaloids. SWIM's FOAF has seen it working with harmine, harmaline and mescaline. (even though other minor problems emerged when these alkaloids were tried). If the jungle spice thing is an alkaloid, then it is very likely to be salted out with FASA along with the other alkaloids.
In two occasions SWIM's FOAF bothered with jungle spice; the first was doing xylene pulls and evaporating it down. This was a mixture of dmt, oxide, jungle spice and oils. This was strong at 10-20mg but it was still a dmt experience.
The second time he used FASA, then washed the freebased alkaloids with naphtha to get dmt out. Maybe he washed too much just to make sure all the dmt gets out. Mix with warm naphtha, then shake the hell out of it and repeat it for 10 times. He was left with an oil which contained the jungle spice(???-he doesn't know for sure since FASA and jungle spice had never been tried before) and definitely some dnt-n-oxide). This product when smoked at the 15mg range had no much effect. Just a flimpsy dmt feeling (due to the n-oxide?)
But it had MAOI-like activity. This was revealed when dmt was smoked some time after the FASA-jungle spice bioassay. This happened at around the same time when burnt reported on behalf of some unknown person that there is some evidence for a beta carboline-like structure in jungle spice. And this is SWIM's FOAF things as the predominant theory. There's something that amplifies the pharmakokinetics of spice.
So, yes, evaporated jungle spice is stronger, but this is due to its dmt content. (dmt has been detected as a component of it using mass spectrometry). SWIM's guess is that evaporated jungle spice and FASA-derived jungle spice are going to be very similar if they are washed very well to get all of the dmt out.