You should never have techs that downgrade a unit. You should only open up the possibility of a new option so the player can decide based on their current resources and situation what would be the most beneficial to build.
Besides, wouldn't such a large reduction in speed already be like it's been disabled?If a tank losing it's treads is such a bad thing, why would you ever design a tank that can barely move even before it gets hit.
Give it a new name or something then ? Have something like "Armoured Tiger" etc appear in Vec fac.
Not exactly the same thing this, but you currently have somewhat the same thing in the game already with the Reinforced Vehicle Construction research (The one leading to tiger research). Here Cargo trucks get extra armour, but you have to have rare in order to build them. If you don't have or lose your rare it means your kinda messed up ?
I'm no expert on this field, but I think the reason tanks today are being developed with emphasis on speed is because of Aerial weaponry. A tank is no match for a airplane or helicopter. Thus it has to be fast to avoid being hit to easily.
In OP2/OP3 there are no aerial units (?), thus a heavily armed vehicle which can withstand a lot of enemy fire without taking too much damage or being disabled would certainly have it's advantages.
Only example I can come up with from the top of my head would be the Panther or Tiger tanks from Germany during 2nd World War. They were so heavily armed most shots from Allied forces would just be deflected or do minimal damage. Panthers and Tigers often served as spearheads in attacks on defense lines, or as mobile defense positions dug into the ground at strategic positions. Allied commanders usually preferred going in wide circles around these tanks instead of engaging them, or only attacked when they had an advantage in numbers and could just pummel the Panthers/Tigers from all sides until they broke down/got destroyed.
No point discussing the pro's and con's of old tanks I guess, but I just wanted to press the point about the Strategical Niche such a vehicle could potentially fill.