Do "Legacy" Engines Have Better Motor Control Than "TMCC" Engines?

The best answer might be for someone to post a video of their JLC GG1 operating under Legacy and see if it performs to your liking.

TMCC engines run better under legacy, but not as good as a legacy engine under legacy.

In addition,in my experience a legacy engine runs better under TMCC than a TMCC engine does under legacy.

Legacy loco has better speed control than TMCC locos. Lionel has also improved gearing on many locos with backdriveable gears as well as some having secondary gears, which was originally K Lines design.

Generally Legacy is a little better, because it has 256 internal speed steps.  However, a lot depends on the gear ratio, and whether the loco has back-drivable gears (i.e., the wheels can turn the motor, like a Postwar loco.)  Lionel added these improvements to some designs when they were re-released with Legacy, and it improved the operating characteristics.  

I don't believe the JLC GG-1s had back-drivable gears or the "Liondrive" power trucks.  At this point, only a Lionel insider would know if the Legacy GG-1s will adopt these features.  My guess is that they will not, which means the Legacy GG-1s won't run much better (or different) from their JLC predecessors.  Just my $.02, i'm not an insider!  

I run my JLC GG1 through my legacy system and it runs fine.  Obviously the fine tuning of speeds isn't there like on a legacy locomotive, but it's definitely not a bad runner once you get used to that.  If you don't need the full legacy range of speed steps or the new vision features, definitely go and save a bunch of money.  I don't see where the features included in the new model make the new much higher price worth it.

Unfortunately I don't have a video showing it at it's slowest speed.

Probably in general, but not universally - see "gear ratio" comments, above.

And speaking of that, my "slowest" and smoothest locomotive is an ERR Cruise Commander-upgraded scale Williams brass 2-8-2, from the late 80's (my upgrade is 3 -4 years old). By direct comparison, it will run slower and smoother than any Legacy loco that I have directly compared it to. That's not many, but beats all of them.

Gear ratio: the most important single thing in smooth and realistic locomotive operation.  Good enough and cruise not required.

For slow speed operation, legacy absolutely takes the gold. With both TMCC and DCS, many engines oscillate at low speed because it takes the tach reader too long to adjust the power going to the motor. Now if only legacy was digital! Then you would be able to speed match locomotives for even better operation...