It is a valid point Leethal and this question had to be asked sooner or later and i will throw in my two bobs.
i don't airbags - so will leave that to others.
i dont think there is too much doubt that the mod is 'technically' illegal.
However, i found the standard seat to be absolute rubbish and created excessive aches and driver fatigue on longer trips. even short trips of less than 30 minutes saw me in pain. trips beyond a few hours were not possible. with the spacers i did Coffs to Sydney with one short stop and, although a bit saddle sore, was not in any undue pain. They make that much difference to me.
So ergonomically the spacers allow for driver comfort, so significantly, that i 'PERSONALLY' have no problem with their fitment as they could easily afford me 'safety' by reduced fatigue. i would far prefer to avoid an accident assisted by an illegal mod than crash into someting with the knowledge i was 'legal'.
From a pure engineering standpoint i would expect most tension loading on a frontal impact is on the rear of the seat base and this is effected only to a minor degree in the angle it attaches. The 'thread extenders' are of solid construction and as a component could provide bigger bending moments on the chassis connection etc and again would probably need some form of testing which is well beyond the resources of us.
This seat comfort is such a big deal to me it would have stopped me from keeping the vehicle long term. The seats are that bad IMO. My only other option is $2500 of recaro per side (one looks stupid). In the end i believe that the use of them is safer for me in providng significant comfort levels for not a huge cost. If they failed in an impact then i probably have other things to worry about as well. i just make sure that all the seat bolts are tight every now and then which is probably more important that the extenders themselves.
it is a shame mitsubishi can't get some basics right. AND before all the standard seat lovers start PM ing me with "what the [censored] is your problem" i understand that for some people - the seats are fine. but their lack of adjustment and MANY others here on this website that have issues it is proof that there is a issue.
This car is like a bad drug habit. It is taking all my money and time, my family are concerned, but new mods just feel sooooooo good.