Ive purchased a spoiler from Grams Styling and the service was excellent. They let me come down and fit it up to the car, gave me advise and plenty of time to think.
The finish was OK, but not the best, so needed a little work. However it was much cheaper than alternatives so not unexpected. It wasn't too bad, just a bit wavy so needed filling and levelling out.
Its a cosmetic part so 'quality' is gonna be down to how well the bodyshop finish and fit the spoiler. If it was a bumper or similar i would be more concerned as they can be too far out to make fit right.
With aftermarket styling parts paying more doesn't guarantee better fit/finish, ive found that some 'branded' parts have pretty average fitment considering the premium. So unfortunately it can be a bit of trial and error
Hope this helps