Yes I looked at those a while back (they are quoted in lines per inch so 12700 is 500 l/mm) I asked them to give me some figures for efficiency like they do with their other gratings. The declined, so I assumed they were not particularly efficient. It would be interesting to know what they are like though.
For a simple grating in an uncollimated converging beam setup there is not really any advantage going to more than 100-200 lines/mm as the optical aberations of the configuration swamp any potential increase in resolution. The disadvantage with using a transmission grating in a collimated setup is the in line configuration means the spectrograph is rather long and rapidly become unweildly compared with a folded reflection grating design. Also the efficiency of transmission gratings falls off rapidly as you increase the number of lines (proportionally less gap for the light to pass through) This is why most spectrographs use reflection gratings. Now if only there was a source of cheap but efficient reflection gratings..
Cheers
Robin