1.) I think you might be missing the bigger potential of a 152 mm cannon, and here's a hint, it's not tank warfare. Take the RPO Shmel launcher. It's thermobaric munitions at a 93 mm diameter has the same destructive potential as a 155 mm HE-Frag shell...now think about that potential scaled up to 152 mm cannon thermobaric shells. At 152 mm, you might see destructive potential rivaling a 203 mm HE-Frag shell, or maybe close to 240 mm mortal HE-Frag shelsl. While it's possible they could do something similar with with the 125 mm, the best thing a 152 mm shell could bring would be both the destructive potential and the extended range.
TOS for ranges up to about 12km in the current version and Smerch out to probably 120km or more... plus coalition...
Certainly no tank gun will be firing at targets more than 110km away.
2.) Now lets also think about the potential biggest threats to MBT's....short answer it's not other MBT's. The two biggest threats facing MBT's as of now (taking in to consideration the recent conflicts in the ME) are insurgents with man-portable ATGM's, and modern MLRS. In my opinion the 152 mm is better suited to fight these threats because of the additional range it could bring, that the 125 mm couldn't. Destructive potential without sacrificing extended range is the main point here.
Insurgents can be dealt with using standard weapons on IFVs and APCs and the roof mounted remote weapon stations on the Armata tank... while their RPGs will be stopped by APS systems.
MLRS will be 40km plus away from the targets they are engaging... Russian MLRSs will be even further away... no tank will be in a position to deal with those.
[quote3.) With a guided thermobaric shells, at 152 mm, you could potential engage insurgents ATGM's from possible stand-off ranges as far as 10-15 km's, and destructive potential nearing a 203 mm HE-Frag shells. You could literally clear out a 5 story buildings harboring insurgents (think Syria, Chechnya), hitting every floor, hitting windows from opposite ends, either obliterating or suffocating them. 10 to 15 shells could potential see a partial collapse of one side of a multi-story building, killing many insurgents and exposing the rest in the process.[/quote]
Easier to just deal with the ATGM using APS and if you want to bring down a city block 12km away, then TOS...
4.) Using some technology from Koalitsiya, stand-off range shells could be developed, maybe with a glide-kit you could see a shell that has a range of 40-60 km's. Because it's smooth-bored, it wont have the 70 km range base standard that Koalitsiya has, so this is why a glide-kit would be necessary. Naturally this shell will be significantly less powerful than the thermobaric guided shell because it will be HE-Frag, and the warhead weight would be decreased as well in favor of extended range. This wouldn't be a problem because this shell's main purpose would be as a anti-MLRS munition, and most MLRS are minimally armored, the main purpose is it's range, giving it the ability to engage MLRS on a more even-footing. Even a UAV could be developed from that shell.
But why fit such things to your tanks? surely the anti MLRS role is better played by the Smerch at 120km plus...
And coalition has guided shells with 70km range already... why waste time giving tanks the capability to do the same?
5.) Don't think the 152 mm T-14 would be the standard version replacing the 125 mm version, think if it as a support version complimenting the 125 mm version. May'be a good ratio would be for every '10' 125 mm T-14's, there should probably '1' 152 mm version, situated behind (may'be 500 m, to 1 km) advancing and forward operating 125 mm T-14's.
Its main advantage is the ability to kill tanks at extended ranges so you would not just put one there on its own a couple of kms behind the other vehicles.
A 152mm calibre gun unit would be deployed in an overwatch position to destroy enemy vehicles when they appear at long range... the other tanks will try to get closer where their 125mm guns will be effective...
The same argument could be said for the 57 mm autocannon...there's already Sosna, Pantsir, Tunguska, Tor, so why do you need the 57 mm gun?
the main gun on an IFV needs to be able to deal with enemy vehicles... its ATGM should deal with the heaviest enemy armour, but the main gun needs to be able to deal with the rest, and the 30mm is not powerful enough to deal with 32 ton NATO IFVS... and air burst 57mm shells will deal better with UAVs than 30mm cannon shells can.
300 meters is within the range of many man-portable ATGM's (i.e. Kornet-E max range of 5.5 km's), you'd need to be in a range beyond 6 km's to be safe, the 152 mm would have that over 125 mm.
A 125mm HE Frag round would be rather accurate as it is direct fired, and can hit targets out to about 10km with a relatively flat trajectory.
If that is not powerful enough then the 220mm odd rounds from the current model TOS should be plenty of fire power.