To get most of the necessary modifications on the BMP-2, it would require around 46,700 RP, which if you get around 2000 RP every match (basically completely stock), it would take you 23.35 battles to get those necessary modifications to perform almost up to par to if it were spaded. Even at 100 battles, or slightly more, those 23 games where you underperform because of it not being a spaded vehicle is a somewhat significant portion of those 100 battles.
But I do agree that once it’s at somewhere around 200 or so battles, the time you spent spading it out, and the subsequent KD that resulted from it, becomes relatively insignificant, like in this person’s case.