Actually its 2% technically since they update both attack and defence. Every time you upgrade the car one you get 1% to attack and defence. So when fully upgraded at level 10 they are around 15% to both car attack and defence.
375 gold isn't bad for 30% mod.
On my device the boosters offer 5%- and if the footprint wasn't so ginormous I might be tempted, or are you talking upgrade? I seem to recall that's why I never upgrade the one booster I have bought.
"When the going gets weird
The Weird turn pro."
-- Hunter S Thompson, True American