Interesting interpretation,
Sprite.
My experience has been somewhat different in that for the "Unmodified" series, all I've had to avoid was adding a Wing. All other modifications seemed allowed (altho' I can't be completely sure as aiming for higher A-Spec values meant not using such things as weight reduction etc).
Here's an extract from my 'diary' notes for the Premium Sports Lounge as an example:
EXTREME HALL
Premium Sports Lounge
Entry Requirements:
Unmodified Cars Only (from a restricted list)
Normal or Sports Tyres
International B Licence Required
Prize Money: 12,000cr/race
Prize Car: Ford GT '05 (Value 31,849cr)
Start Type ----- Circuit ----------- Laps
Rolling ------ Cote D'Azur ---------- 4
Rolling ------ Opera Paris ---------- 5
Rolling ------ Hong Kong (R) -------- 5
Grid -------- High Speed Ring (R) -- 4
Rolling ------ New York (R) -------- 3
My first run at this was in a fairly highly tuned Aston Martin DB9. The A-Spec values reflected that.
{edit out lots of race specific notes}
Not so long ago, I re-ran the series trying for more A-Spec points. I used an Aston Martin Vanquish with the following hardware modifications:
S3 Tyres
Semi Race Exhaust and racing Chip
Brake Balancer
Semi Race Suspension
3-Plate Clutch, Racing Flywheel, Carbon Prop Shaft, Custom LSD
{clip off about three pages of further information not relevant here
}.
Essentially, it appears to boil down to that you can use any production car in these races as long as you don't have adjustable downforce.