For now, I believe the T58 is good, but it is in an awkward situation where if you move it up and it sees stuff like T-55AMD, Chieftain mk.10s , PGZ 89, KPZ-70 or T-72s it’s not going to fair well
It is strong, 2.6 second reload + massive overpressure HESH/HEAT rounds, trollish turret armour and people being unfamiliar with how to deal with it.
But it also has a lot of disadvantages as well, lack of stabilizer, slow velocity, terrible ballistics, relatively slow, chemical rounds as the main ammo while the AP is only 199mm pen, and it is at best M103 armour, meaning it is not going to do good in open maps, against tanks like 59D1 or Olifant, with has laser range finder, stabilizer and DM23 dart (which are more than capable at going through the hull armour and hitting ammo, or sniping weak spots just under the gun.
I believe currently it is just people being unfamiliar with how to deal with it, while its armour is just good enough at dealing with full caliber rounds (BR-412), low pen HEATFS (320mm ish pens) or early APDS (DM-13), making it seemingly very tanky. People keep shooting the turret cheeks expecting it to blow up like AMX 50 Sub or Somua, or the weirdly angled upper turret sections, of course it will seem like it is almost invincible.
After giving it a try, the moment someone with DM23 come around, or people who know where to shoot it with a DM13 shot, or you get a proper flanking shot into the hull , it results in ammo blowing up most of the time. While the breech is quite easily knocked out.
Turms III is a different matter, due to speed it could reach and flank spots where T58 can’t, despite the 105mm loads at 5 second beside 2.6, it has APDS that could go through higher BR targets more reliablely , a 30mm cannon to kill soft target without wasting main caliber gun, while fully stabilized.
But, at the same time it is hard countered by anything with Autocannon, be it a sneaky warrior in a bush, a by passing BMP-2, a XM246 or XM800T, and the 8.3 BR range is saturated by these. It has a higher skill floor compare to the T58, making it very skill and luck based , an encounter against a autocannon tank is solely up to who shoots first as it has no armour to rely on.
I’d say both tanks are difficult to balance; the Turms III could go to 8.7 at best, but the main reason we are having to conversation is solely due to compression.
Turms III and T58 meeting 7.3 /7.7/8.0 tanks is a total slaughter, but when fighting a 9.0 or 9.3 tank, such as if a Turms III meeting a XM803 or T58 having to fight a T55AMD, then it is the reverse.
I think purely in 8.3-wise (without considering up-tier or down-tier situation, 8.3 is okay-ish balanced compared to most other BRs. But even counting in 7.3s and 9.3s, the compression makes any balancing action awkward