What is it you're wanting to beef up ?
I would retain utilizing a plastic spur gear...
especially if the slipper were removed or locked.
Something may need fail...
and a plastic spur gear is inexpensive and easily replaced.
Damage of other drive line components will cost more if they might break.
And are not generally as easy to replace.
When you beef up one drive train component...
it will likely just find the next weakest point and break there.
So are you attempting to build it up as being bullet proof...
and willing to invest a small fortune ?
Or just trying to make it strong enough to hold up to your brushless motor ?
I run the Axial steel gears in my Axial transmission(s)...
https://www.amazon.com/Axial-AX30708-Locked-Transmission-Wraith/dp/B00GALTG80
And they have proven to be strong/durable/reliable.
Everything else in my Wraith drive line is stock original,
including the Trans. outdrives (outputs to drive shafts).
And tho' I have upgraded to the HD WildBoar drive shafts...
I see no reason to go with metal drive shafts and beefier trans. out drives...
unless maybe I were running a 6s LiPo.
I'm not a fan of the RRP trans. outdrives and their set screw DS install.
Tho' their 6mm shaft is stronger then the originals...
I've yet to break a stock outdrive running on 3s.
If you use them... use blue, maybe red locktite on those grub screws.
Granted I'm not running brushless...
and so have no idea how much more power you're putting thru your drive train over a 3s LiPo.
But both my SCX-10 and Wraith have held up admirably to 3s Lipo use without any problems.
Both utilize the HH's BRXL ESC, HH's brushed TorgueMaster 35T motor.
And are geared (48P) 87/16 (SCX-10) 87/14 (Wraith)
Have never stripped a plastic 48P spur gear if the tooth mesh is set properly.
And the spur gear cover is retained.
Plus the 48P gear mesh runs quieter then does 32P gears.
Obviously I'm not a speed racer nor basher.
Now if I were running 6s LiPo in either of my crawlers...
I would probably need to beef up everything in both drive lines.
Tho' it's likely things would still break or simply wear out.
For example... if a bearing were to fail inside the transmission...
it's plausible that the internal steel gears could become damaged as well.
Not sure what to advise you on beefing up...
unless you intend on doing all of it.
And that is gonna become costly to do.
A single beefier upgrade is not where it likely ever stops. jm $ .02
IMO... it's probably best to beef up only what breaks or wears out.
And save yer money for that rainy day.
But if you want bullet proof...
you will need spend a small fortune to get there...
and it could still break /or wear out
requiring a higher expenditure to replace/repair.
For some... the expense of bullet proofing is of no concern.
