With regards .410 slug.
As DM pointed out and Sandgroper also knows, I looked at bringing the T14 over for this reason, suffice to say there was an underwhelming lack of support from some areas, extreme prejudice (especially from one police region who shall remain nameless) plus a few other factors that led me to say

it - not worth the hassle.
The UKPSA thing was thrown up time and time again (I think they were contacted but since it wasn't a 12/20 they weren't interested ) and in short, it was"not proper" as only 12/20g was "suitable..It would not count in Comps but a whole UK wide series was devised for it and to be fair some clubs did show a good interest whilst others could not see past their 12bores...!
The wide differences of opinion and acceptance from the police was scandalous..talk about paranoia..There was also some to-do about ranges..seemingly a range suitable for a .308 was not suitable for a .410slug...
As I said, a whole competition was designed, the plan being to run it across the UK with a special deal being available on the T14 e.g extra mags, box etc etc.. - actual prizes were planned etc etc . As I said, I binned the idea due to being screwed around at both ends!.
As for the .410 / 36cal slug, well it packs a wee punch compared to a 12g slug which is like an aerodynamic fishing weight. As for knocking over a steel plate, depends on how the plates are set up.
I use my T14 here as a substitute for the 5.56 AR semi auto I cannot have - but the T14 is seriously ammo sensitive as it was not designed around a specific cartridge. IT is not able to feed just any make or type of .410/36 - do not listen to anyone who says otherwise.
A T14 is NOT necessarily a .410 so get that clear if you ever want to buy one. The UK ones at the time were dual stamped 36 cal (down to the makers not understanding the UK system) / .410 and that caused a LOT of confusion and bad press so to speak. They are NOT the same thing when it comes to the T14....period...no matter what is stamped on the barrel /receiver. The mags are different for one thing and they will have serious feed / eject problems. In effect .410's went to the USA, 36cal came to Europe - even then there were ammo feed issues.
Here's a piccy of the effect of a .36cal slug fired from a T14 (the slug is YFS Turkish make) on 5mm steel plate compared to a 9mm FMJ 115gr round fired from a Beretta 92 - both at 25 yards
The barrel length would of course affect the velocity.....My T14 is an "M4 alike" with an 18" barrel and milspec polymer upper and lower..built for me by Safir. These are of course not on in the UK and the T14 would have come full UK spec.
I can't post a piccy of my T14 due to this pixel size stuff, what's wrong with a FILE size christel..easier for non techies like me.
Anyone wanting piccies email me and I'll send them.
In a nutshell, the .410 Safir WOULD have been a good fun gun for younger shooters and those of a smaller stature, that was the attraction of it. However there are alternative .410's that, if I were going that route I would look at in the UK - primarily due to the ammo issue.
Hope that helps.