Crew and/or ammunition arrangement?
4 man crew with no hull gunner, so a 3MT. Ammo? Unlikely. The T-34 had about half of its ammo in boxes stacked on their narrow but long edge on the hull floor and to get to those the turret crew had to remove the rubber mats that they stood on to get to those boxes. The T-44 with its torsion bar suspension and wider but lower hull had more normal ammo racks. If anything ammo retrieval should be faster.
The 100mm naval gun mounted on the SU-100 and the piece in the early IS is one of the great Soviet missed opportunities of the war. Why it lost out to the 122mm gun is a story that deserves telling.
Supply! The 122mm D-25 (manual screw breech), D-25T and D-25S (wedge semi-automatic breech) was based upon the A-19 corps artillery piece that had been in production pre-war. The ammunition was also in plentiful supply.
The Soviets experimented with a 107mm M-60 gun in a KV-2 turret as part of the development of upgunned KV-3 and later KVs. This was a redesigned stretching of an earlier Tsarist 107mm gun, but it appears that the Soviets decided that such a calibre did not justify the effort and concentrated on 122mm artillery. At some stage they decided that they wanted a 100mm as an AT gun and given that they had the excellent S-34 100mm destroyer gun and some manufacturing capability, a version was designed as the D-10. However it appears that volume of both gun and ammo was insufficient for IS-2, ISU-122 and SU-100 at that time. The IS and ISU series were intended for breakthrough operations rather than antitank work and the D-10's HE round was 15.8 kg and inferior to the D-25's 25 kg. Tanks fight infantry, artillery and bunkers more than other AFV, indeed typically WW2 tanks only carried about 1/3 AP rounds with the rest HE or smoke.
They also had the excellent B-13 130mm destroyer gun, probably the best destroyer gun in the 5 inch range in WW2. This was put in a number of post war experimental heavy tanks and SPG, eg IS-7 as well as a single gun in the SU-100Y based on the T-100 chassis. That should easily equal or exceed the German 128mm gun.