yea, i believe that i have seen an interview or somthing(maybe one of his clinic sessions?) where he listed his pedals. He said something along the lines of, " I am more of a just plug in and play guy, but I use this -"
You don't have to boost it, but in studio settings, using a TS9 as a front boost does tend to brighten and tighten up the signal going in a bit, making the input attack a little sharper. It's pretty common practice in 'modern' metal.
You're not boosting the amp, you're boosting the guitar. I shoulda worded that differently.