Andy,
I have never had much of a problem with the API tests, but I don't have any experience with other test kits and have never dipped my toe into the salty side of things. What issues have you had with them? The only thing i find is that the colours of the results can be open to interpretation a little in different lights, but I have no idea if this is any different to other kits.
Cheers
Squid
Problems I had:
The colour chart is awful
Reading 160ppm on RO+ salt
Reading 160ppm and 0ppm on two samples taken from the same tank and tested at the same time
Always reading 160ppm on tap water and tank water
Nitrite and ammonia readings on RO + Salt
The above happened across two test kits (one I bought on its own, the second I got with the SW master kit). Tropic Marin tests are far better.