well... you are asking for an always controversial question! jeje...
Are you from USA? if you are, then you´re gonna have the answer fast and correct...
If you are not, like myself... everything depends on what kind of paint brands your place sells... We all know the correct colour is a black that is not totally "mate"... i dont know if in english mate is the same as here... i think so...
The problem i had with semigloss paints is that there´s nothing to asure you you brightness level is going to be the correct one... I´m from Uruguay and here in the third world, getting the latest technology on materials is not that easy so i really can´t tell you a fact in this matter...
What i´ve learned over the years and the restored cars is that:
1) the best finishing product i can get is PU, 2 components paint... It is the same autobright paint almost every car uses when the exterior colour is not metallic... It doesn´t need clear coat and yes.. basically is the same chemestry as the clear coat, only it is not clear, but coloured...
2) the closer to original i went was with a sinthethic, 1 component paint... 10 times cheaper than the PU but 20 times less resistant, however, the finishing was correct.
3) I acutally owned a 1966 mustang with the original engine bay paint in surprincingly good condition and that´s my point of comparation for this matter... The paint quality Ford used in the engine bays was the same as it was used on the exterior panels, only the color was in fact, a semigloss... What i can say is that paint looked really black, but with no shine at all... Easy to imagine but hard to accomplish as any paint turns grey when the bright is turned off...
Conclusion... My car has a slightly more brillant finish in the engine bay... it is a PU 2K paint in semigloss base. It is made as mate as it can be done with a semigloss base and still it is too bright. The colour is right and the dust of the daily use do the rest in order not to get awfull brightness there...
After i painted my engine bay i did some test in order to get a black mate, just like the finish you see on any black plastic... telephones and so... This is where i want to go with my next engine bay paint, actually it is going to be tested on a 350 SLC Mercedes Benz i own.. For me, it should be the best option for an engine bay and it should be the closer i can get in a mustang engine bay...
From now on, i´ll be using this PU 2K paint for the engine bay as it is the only paint that doesn´t get removed when petrol or break liquid leaks in it...
Hope it helps!!!