It affects acceleration, top speed is unchanged, how quickly you reach it however is changed. What this means in handling term, is that movement translates into momentum quicker, making the dropship feel more responsive and faster, what used to take a second (for examples sake) such as tilting and thrusting hard to move sideways, will now take half a second (again for example, I'm making the numbers up for ease of writing), the top speed you can reach will remain unchanged. Scout LAV's are the best example of this, a abron with no jovians will have the same top speed as an abron with 2 jovians, however the faster acceleration of the 2x abron will make it feel more responsive and thus faster than the other.

It's like 2 cars capped at 60mph, the 0-60 might be different but the top speed remains unchanged.

I have made extensive use of jovians on all vehicle classes, I have noticed no difference in top speed between vehicles with jovians and those without.