James used Marshalls on KEA and RTL (with a distortion pedal) but from Puppets on used the Mark IIc (with the Marshall as a power-amp), then Triaxis, then Rectifiers.
That's not to say that I don't think you could get close with the SL+, I was using it for my scooped rhythm thrash style sound and it did what I needed...but I wouldn't say it was the amp getting the sound so much as what I dialed into the EQ, the rest of my gear (6L6's in a Mesa 2:100 power amp), and my playing style/response. I mean, the thing worked equally well for playing Slayer and Megadeth but we all know that's three different amp sounds.
Basically, I'm saying that with any high gain module, you can get close enough to most scooped thrash sounds that you can work with it to sound like what you want. But for the authenticity, you're not going to get Master Of Puppets from a Marshall tone.