Crap, I wrote a long comment and accidentally closed the tab. Well.
In a nutshell: 15 ramp spells is doable. There are already 15 colourless rocks to fulfill this order.
So the analysis above doesn't take average mana value into account. It simply looks at the (subjectively) acceptable starting hands and goes from there. You can indirectly manipulate the assumed average mana value by modifying which hands to accept and which to mulligan.
I have experimented with a variable MV setup... But the results are pretty wild. I'll return to you if I ever get around to doing some more of that. (Yeah, that's going to be tonight lol.)
Aye, I was just having fun with photoshop. Not exactly recent news, heh.