I am currently using a hybrid of T1 Firewire 1,30 mm mains / Pro's Pro Red Devil 1,24 mm crosses. It makes the stringbed a bit plusher compared to a full T1 Firewire setup, since Red Devil is softer than Firewire. That being said, I have never observed any arm discomfort with a full setup of Firewire. It seems to be one of the more comfortable offerings among the stiffer poly strings.
The main reason why I switched to the Firewire / Red Devil hybrid was notching. With its three sharp edges, (a full bed of) Firewire causes considerable notching after about 2-4 hours of playing. Let me say that the notching didn`t cause any durability issues for me, in fact, Firewire lasts 8-10 hours even on clay courts with heavy topspin hitting. However, around the 5 hour mark, the notching usually begins to hinder string movement, and I feel that the mains are somehow locked in the grooves of the crosses. Switching to a slick round poly like Red Devil in the crosses solved that issue for me.
Spin and durability have not been affected by the softer cross string, at least not that I could notice. I usually cut my strings out after 7-10 hours of hitting, so longevity hasn't been an issue for me. The spin level of the hybrid is great, as it is in the full Firewire setup.
I let some other players at my club hit with my setup and they liked the blend of power, control, spin, ball pocketing and comfort.
I still keep some of my racquets strung with a full bed of T1 Firewire though for competetive matches, in case I face a hard hitter against whom I need a maximum of spin and control to reign in the power of the incoming balls.
Hope that helps.