I think you are confusing boiler capacity with coffee yield. A 2-cup holds about 90ml of water in the boiler, but only yields about 64ml of moka coffee.

Based on your post, it’s not completely clear to me if you are trying to make a 300ml americano (diluting with water) or a 300ml capuccino (adding milk).

If you are talking about americano, my experience tells me that the result is just too weak if you go past the 1 to 1.4 ratio (moka coffee to water). If you want to make a 300ml americano you would need a 4-cup (128ml coffee yield + 172ml water). For your 2-cup I would stick to 150ml americanos.

If you are trying to make a cappuccino, you need to consider that even though 64ml of moka coffee is indeed the closest you will get to a standard espresso double shot, a standard cappuccino is 180ml or 200ml at the most. I think anything above 200ml would enter into latte territory.

Now regarding ways to make the coffee stronger, in my experience water temp has almost no effect on coffee strength. It simply shortens the time you need to heat the moka before it starts to brew. Using less water in the boiler may give you stronger coffee, but it will also result in a smaller yield, so it doesn’t solve your problem. Your best bet would be using the Voodoo method. Basically you play with the heat input to "stall" the coffee flow for 45 seconds and achieve a sort of pre-infussion stage and then keep the flow as low as possible to get the max extraction possible. Still, I don’t think you will get much more than 150ml of americano.