I do not know if this is going to help you or not, but I was in the same position as you 2 days ago.
I had a friend who had an already softmoded HD with Splinter Cell and I had a 1.1 Xbox with a matrix modchip. I read in another post that you can flash the TSOP with a softmod (did not specify wich one) and with the evolution x flasher as long as the TSOP were not a Winbond.
I prepared a X4981 bios with xboxtool (did a 512 since I installed a switch) transfer to my friends HD, Unlocked his HD and placed it on my Xbox, Then locked the HD to my Xbox and removed the matrix modchip.
I turned on the xbox with the switch on (as posted in the swhitch tutorial) and just flashed the TSOP with the evolution flash utility (was as easy as flashing my matrix modchip).
So as long as you have an already softmoded Xbox, and followed the tutorials to prepare your Xbox soldering points, switch, and proper Bios depending on your xbox version, you can flash it with the evolution x flashing utility as long as your TSOP is not a Winbond.
But just to be sure wait until a TSOP flashing expert in this forum agree or disagree with that statement.
And Good luck with it.