IMO, they just can't produce enough low-mid for a car environment and there will be a large "hole" between the comps and the sub. Subs sound best when they are only running below 100 hz (
In a quiet recording studio small drivers can sound great and get surprising low, but in a car they can't overcome road, wind and engine noise.
6 1/2" comps have a natural advantage in low frequency output. If you can make them fit, go for 6 1/2". 7" or 8" would be even better, but they can be difficult to fit in most cars.