Heh heh. Yes and no to all the above.
JL chassis are not quite carbon copies of the original Tjets. First of all, the gear ratio they come with is the same gearing as Aurora Tuff-Ones, not Thunderjets, which gives them a significantly higher top speed and makes them a little harder to handle. Neither is all that finicky, and both use the same tuning principles. The JLs will respond to tuning and tweaking--your dragging-a-brick Camaro could possibly be adjusted and tuned to run nearly as well as the Mustang if you have the time to put into it. Areas to look for problems: Crown/pinion gear mesh, brushes binding in the brush holes, bent rear axle, I'm sure someone will chime in with more. Personally, I've had problems most frequently with crown and pinion gears being out of round, and I've had good luck sanding the crown gear boss down to give it a little more room in the chassis (to make up for pinion gear wobble) or in extreme cases replacing the crown with an Aurora gear.
Many of the parts will interchange... it's getting common to put JL magnets and/or armatures into old Aurora cars for more speed. (The magnets are a tight fit--sometimes a little grinding is needed.) The gear plate won't interchange, but most of the other stuff will. I bought a bunch of JL wheels/tires at a slot car show not long ago and I've put them on some of my original Auroras. JL brushes are actually copies of Aurora's AFX brushes, not Tjet brushes... some folks say they're better for performance, others feel they wear the commutator faster. Early JL pickup shoe springs are too strong and contribute to lifting the pin out of the slot under acceleration, and Release 1 cars had too-short guide pins.
For the most part, I prefer original Aurora chassis because they seem smoother when you get a good one. I also find the top speed of the high-geared JLs makes them a little twitchy to blast around the track. However, I just got 10 of the chassis only for like $4 each (to add to the 25 or so complete JL cars I already had) and am having fun doing different things to them--I have a couple geared down to original Tjet gearing, a couple with skinny Tjet wheels/tires, a couple with heavy weighted brass front wheels on them to help handling, and there are one or two that just feel like they are "dragging a brick".
NOS Auroras can be had for 8 or 9 bucks a pop, sometimes less if you buy 10 or more. I've seen JL chassis cheaper than that on Fleabay, maybe $5 or $6 each, and as I mentioned, I recently found them for $4 from a guy who had bought them bulk. However, I'm saving up the slot car budget to get more Auroras, if that tells you anything...
Have fun. Try stuff. Keep asking questions. I'm sure lots of other folks here will have stuff to say. :wave:
--rick