Yer thinking a bit backwards. 15/50=65 teeth.
15/52=67 teeth. More teeth, more chain.
The limiting factor on whether or not you can squeak by with a stock length chain when you have more sprocket teeth is how far forward you can shove the axle. If a stock chain is too long for your chosen combo it becomes how far you can set the axle BACK before you run out of adjustment. As long as it's only a couple tooth difference you usually won't have to sweat it. When you get to a 2-4 tooth difference it can sometimes be a game changer depending on how you arrive at your chosen ratios. Front sprockets are cheap, rear sprockets are not, and chains cost much more.
Theoretically you'd want a 124 for that combo, but in reality you may get by with a stock length chain if there is room on the adjusters.
15/50=3.33:1 final drive ratio. 65 teeth
14/47=3.35:1 ' ' ' 61 teeth
Nearly identical ratios, but the 14/47 MAY require a shorter chain.
15/52=3.46:1 ' ' ' 67 teeth
14/50=3.57:1 ' ' ' 64 teeth
Close enough to the same ratios, but your 15/52 MAY require a longer chain.
So you see you're only slightly higher (lower numerically) than the stock setup. Slightly better for chain wear but not much change in rpm, if that's what you're after. But you'd most likely need a new chain for your combo and it nearly duplicates stock, whereas changing only the front sprocket (cheap) to a 15 results in a significant drop with only a one link difference in the chain.
