There are a few factors that determine how "metal" it will sound - poweramp, speakers, pickups etc. But yes, you can certainly get a cutting metal tone out of the Triaxis. It has a lot of gain available. The only limitation might be the amount of EQ, but the dynamic voice control gets you a fair way (it can make the typical scooped-mids shape which is used to create a metal tone on the Mark series 5-band EQ). You could add a graphic EQ in the FX loop.
It's not a Mark V though - you can't switch in the rectifier diode sound unless you have a poweramp that does it. The Mark V has more options in total, but the Triaxis offers more options that are easily accessible using a MIDI controller. It really depends what you want it for and how many sounds you need to be able to access quickly. For a studio or gigs that only need a few sounds, get the Mark V. For live use requiring many sounds, you might prefer the Triaxis if you like fully MIDI controllable setups.
The Triaxis has been used on plenty of metal albums, such as early Dream Theater, Metallica, Jeff Loomis (Nevermore). It can be very effective if you understand the other components that create the overall sound.