TMs already have steel pinions,and pretty decent ones at that. Standard sintered+machined pinion,just as strong as any old stock pinion.
Thing is on TMs is that once you upgrade,you have to upgrade nearly everything and do it very well.
Once you replace the zinc sector gear you have to replace the other gears since the steel sector will chew the weaker steel gears up. Then you need to replace the piston since it's ABS and will get wrecked by the steel sector.And you have to do a good job at all that since TM gearboxes have very small margins allowing for error. One cockup and the whole thing grenades itself bit by bit so you have to replace any parts than can be a source of failure and tune everything correctly.
That said not all TM parts are bad. Tappet plates are fine,cylinder,cylinder head and nozzle a good quality and seal well,hop up units are well made and Pistol grips give the best motor alignment. ZCI and TM pistol grips are the top choices for 'silent' gearbox build where perfect motor alignment is needed. I've found lots of pistol grips are cast shodilly so the motor alignment is off giving an irremovable screech.
TM seem to have improved the quality of the gear sets & pistons on recoil shock guns. The sector is still pot metal and does like to be chewed up under heavy stress but the spur and bevel are partly machined sintered carbon steel which is pretty good,they like to mesh well with other gear sets too. The pistons are much much stronger,and will not fail unless you jam in a very strong spring (m120+ above) or really cock up AOE correction.