I'm right there with you, robstyle. I'm an old SoCal car modder myself. There were always reasons for cars to continue to be modified though, whether it was because they got wrecked, or blew a motor, or newer technology became available that opened up new possibilities. Maybe you got a great deal on a non-EGR JDM motor that was rated higher than the domestically available USDM counterpart. Usually, it was as simple as that never-ending desire to just keep making them go faster. The faster they were, the more often something got broke, and then replaced with something better.
I would have to disagree on the garage-queen status for the Falcon, though. Apparently, Han went right back to smuggling, and continued to do so after the Falcon got stolen. The history of the ship didn't seem to matter too much to those who stole it, as it ended up in a junk pile on Jakku, and was regarded as "garbage" by Rey, despite her otherwise intricate knowledge of the ship, and never realizing that it was the Millennium Falcon. If stealing it was about bragging rights over having a famous ship, it would have been blabbed.
Maybe that is the only reason it did stay so unchanged: despite being valuable enough to steal, it always ended up regarded as garbage on closer inspection, and left to rot over the years.
That's why I would expect to at least see more damage and/or repairs, and with whatever was handy and available.
We saw technological and design advances on everything else by TFA. I would expect to see some upgrades on the Falcon too, assuming she was still a useful ship, worth getting stolen multiple times. Since nobody seemed to realize or care about the historical significance of the ship, and we know how unreliable the ship was, I would expect that the various owners would have at least attempted to do more with her. Likewise, if the ship did have the reputation and was also recognizable, I would think that somebody who stole it might be likely to want to try to disguise it as well.
Personally, I think the extent of thought that went into the "compressor" was to come up with a plot device to establish Rey's aptitude to Han, and nothing more.
I'm totally with you on Howard, and the car chase film possibilities. I totally hope you are right.