Hello and welcome to the GTP.
Regarding your questions...
I've never heard of an extra prize that goes with any race in the game (except prize cars maybe but I don't think of those as "extra").
Also, since there is no extra prize switching to B-spec will not make a difference.
However, if you do switch to b-spec you will not get any A-spec points.
Also, I'm thinking about this bonus idea...
I'm almost 100% sure there is no bonus for doing a race all the way through in A-spec.
However, I'm sure you will get prize money for winning the race (regardless of what "spec" you raced in) and you will also get a prize car (regardless of spec as well).
So, sorry to say it, but there is no bonus for doing a race in A-spec only.
But atleast you will still get the prize money and the prize car. 👍
Hope that helps (even though it is probably too late to actually help).
And once again... Welcome to the GTP.