just flew a diamondback.
if its stripped down its fairly similar to the stripped down courier for speed.
Boosts to 441.
When you have A equipment except for the life support and sensors its quicker.
Boost around 420 without cargo.
Its weapons are more central too, so if you like fixed weapons the diamond back is better.
The two wings of the courier aren't that far apart, buts its enough when fighting something smaller than an asp to be a pain to hit with all 3 weapons at once.
I'd say the courier is an upgrade to the viper, its got better shields and can be faster with more power available, and more internal space.
Couriers exploding everywhere here

, ship ramming at stations seems worse than before the changes too.
*edit, I'm getting over 20LY jump range on the courier too, so thats also better than the viper. If you like the viper you will like the courier. If you don't like the viper, the courier is probably skipable.