The one good thing is they both use 5 1/2 speakers. Which should help imaging.
All I can say is give it a try. But, if it were me, I would upgrade the center speaker without a doubt to the Mythos 10, the center speaker that was recently introduced.
ask about it in the def tech owners thread
Definately go with the Mythos Ten.
This is what I did and my front soundstage has never sounded better.
I replaced a Mythos Eight which was good but just couldn't keep up
with the ST's. The Ten is a perfect match for the ST's.
Display: Pioneer PRO-151 60" Elite
Blu-ray player: OPPO BDP-93, Sony BDP-S1000ES
HD DVD player: Toshiba HD-XA2(2)
Processor: Onkyo PR-SC885
Amplifier: Emotiva IPS-1 150Wx7
Game Console: Xbox 360, PS3
Speakers: Mythos ST(Fronts), Mythos Ten(Center), Mythos One(Rears)