I think aero parts in GT6 are as follows, front spoilers, front canards, rear diffusers/underbody with rear exit, rear deck spoilers. Coupled with more rim styles which can be fitted to every car does allow for a huge increase in customization.
RM is not an aero modification don't assume it is, RM is changing the car completely. It takes a street car and removes the interior, acid dips the chassis, welds a lightweight full roll cage to the interior, tunes the engine to race spec(cams, headers, sump size, etc.), changes the exhaust(shortens it to exit in close proximity to the engine, adds a race prepped lightweight carbon fiber body with air channels(to allow better air flow beneath and over the car). Bigger brake discs are installed, Dot brake fluid and master cylinder are also changed for race spec parts, fully adjustable front splitter and lip spoiler, canards and brake ventilation along with high downforce spoiler specifically matched to the car are all applied. That is an RM car in GT5 and possibly the TC cars as well although some look identical to their stock counterparts like the Challenger and the RM of the 2010 Camaro is a TC. Seems PD only called them TC because they didn't add them to the game as RM variations, which would have made more sense but I guess same guy who leaves the SRF was in charge of that decision too.
GT for some reason doesn't actually show you what happens when you increase or decrease downforce on a race car, it's not just the spoilers that are adjusted it's also the brake ducts and air channel port you see on the car in order to keep the car balanced. We'll probably never get to tinker with such things but hey there is a possibility we'll see some deeper tuning brought back.
Honestly I am just stoked that all the cars in GT5 are coming back since I didn't want to part with any and I really wanted to tune each one completely, many couldn't get front downforce and a few weren't able to get and aero attachments at all. I certainly hope they can in this game as long as the number of those that can't receive is smaller than it was in GT5 I'll be one happy camper. I hope the new aerodynamics model is sweet, hope they also fix that ridiculous draft and fix the NASCAR clean air dirty air drafting which should affect all cars the same but more so in NASCAR, also fix bump drafting it doesn't work well at all.
