The Grille 15 doesn't seem to be a worthwhile tank since the nerfs it received. Just from looking at the stats it's almost like a direct downgrade from the WT Pz. IV from either of the top guns (128 and 150) (except in dispersion and aimtime).
The WT Pz IV has better soft stats with it's top 150 than the Grille 15 does, the Grille 15 has a turret traverse penalty of .38 vs the WT IV's .15. More than 2x the penalty for very similar guns. The difference between dispersion penalties is larger if you compare with the WT IV's top 128
The DPM difference between the tanks with their respective 150s isn't much. However, as most people play the WT IV with the top 128, you lose 467 damage (provided you use a rammer in the firepower slot on both) when going to the Grille 15.
I suggest that the Grille 15 gets a 128 gun that does 560 or 600, but keeps a similar reload to it's predecessor. If the gun receives a 560 alpha, the reload should be slightly better say -.35s off the 100% crew reload giving it a DPM of 3119. This would bump it to above Foch B but slightly below the STRV-103B. If the gun got a 600 alpha, I'd say that adding about .5s to the reload would be sufficient, though I can see it being as high as .75 . This DPM would come out to about 3098, meaning that either of these would be viable options to the 150.
The penetration and accuracy on this gun should be slightly worse than the 150. I'd say 265 pen for the 128 standard ammo would be good enough and .29 accuracy would be fair. The gun handling should be about the same as on the WT Pz IV, though adding .03-.05 to each of the stats would be enough to keep the gun from being a snapshot god.
In the highly unlikely event that this is implemented, WG should take note of which gun the majority of players choose.
Source: https://www.reddit.com/r/WorldofTanks/comments/mksc7r/idea_for_improving_the_grille_15/