Yes it's capped at 100,000 vehicles. I saw a British TV video panel discussion about the deal yesterday and they said that last year they had sold 102,000 here, so the deal covers almost all they would be selling here anyway. So it doesn't seem like it should affect US manufacturers much at all.
US manufacturers will lose market share because US auto market will shrink from higher prices, and relative appeal of used cars. UK can meet its quota by bringing in cheaper cars (mini coopers).
I'm sure the car companies know more about how it will affect them than I do, so I believe what they say and defer to that.
When the panel discussion I saw said the 100K cap is not an issue for the UK since they don't sell much more than that here anyway, I extrapolated that to mean it wouldn't change the status quo for the US side either, but that was just my own speculation.
Fair enough. Though I expect Mangolini will give the auto industry more breaks once they kowtow to him enough. That's what it's all about, getting people to pay up or do other "favors" for him in return for him reducing the amount of pain he's inflicting on them -- "nice business you have there, be a shame if anything happened to it".