WG main problem with the 263 line as far as i can tell is just the tier 9 SU-122-54, because its not a rear casement TDs. The Su-101 is already basically 2 tanks in 1, break the SU-102 off into the tier 9 slot.
SU-101 - Powerful 100mm gun with high DPM and a mediocre 122 option.
SU-102 - Powerful 122mm
OBJ. 263 - 130mm