This would be great to see. I personally would prefer for the M551 we currently have to be upgraded to this M551A1 TTS variant.
As others in this thread have pointed out, the base M551 is inherently difficult to balance. Adding the M551 TTS as a separate vehicle would still leave the base M551 as it is, and the years-long issues with balancing that vehicle would remain. It is, fundamentally, awkward to use: poor armour, a long reload, and a choice between hard-to-aim HE & HEAT, or a mediocre ATGM. The rangefinder would be a godsend for the M551, and the thermals would be a nice bonus.
An 8.3 M551A1 TTS wouldn’t have anything out of the ordinary for that BR. Hell, it wouldn’t even have anything unusual for the US specifically at that BR. The M60A2 has the same gun and an LRF, and the M3 Bradley has the same thermals as the TTS, both at 8.3. It’s not that unusual to find thermals on 8.0-8.7 vehicles, and the Sheridan is, comparatively, less benefited by thermals than other vehicles in that range with better guns or optics.
The M551 could often struggle at 8.0, but its advantages in downtiers made reducing its BR out of the question. Now at 8.3 after the decompression changes, it struggles even more, but a return to 8.0 wouldn’t really fix anything. It has fundamental flaws which make it less suited for this game than many other tanks, and BR changes can’t really get around that.
An LRF would dramatically help the tank’s basic usability, which is one of its biggest issues, but even with thermals it would still probably be a worse pick than the Bradley most of the time. Turning the 8.3 Sheridan into the TTS wouldn’t make it OP, it would just make it better at the one niche it has.
The HE on the Sheridan is used so little I think most people forget it has it, but on the M60A2, it’s usable — even fun — with an LRF to make sure it lands on the small spots it can actually penetrate. An M551A1 could do that too, and while it would be dramatically worse at it than something like VIDAR, it would at least have something to help it stand out from other light vehicles at its tier (besides its current overwhelming awkwardness).
I would take it as a separate tank, but what I’m really hoping for is a rework of the M551 into this version, which fixes some of the tank’s inherent problems and would be easier to balance.