Not much more powerful. The current most powerful mobile chip is the Tegra X2, the Switch has the Tegra X1. So even at $100 more it wouldn't be a significant difference, Also if you care to look inside the Switch the battery takes up roughly 60% of the interior of the device leaving little other room for the SoC, heatsink, and fans. Still the device using a more powerful chip would draw more power, and thus produce more heat, which in turn affects battery life. The Joycons wouldn't effect this because they are mainly standard bluetooth input devices, the vibration tech might be something of note however.
They were as efficiency as they possibly could be considering the innards and size of the thing (which in itself is slightly larger than a half of the upper screen of a 3DS XL
The only solution to have more power would be to increase the size of the device, which means it loses it's appeal as a portable. The Switch already is on the upper limits of reasonable size for a handheld, anything larger would defeat the purpose and then it brings it back to the rationality of being a handheld form factor rather than a dedicated set-top box like console device which could take advantage of more power.
Ultimately it comes down to the current progression of mobile technology, which is mainly limited by battery itself. The Tegra K2 is a decent chip, but it's not going to make a world of difference. 2.5 Times more GPU power is basically the difference between the PS4 and PS4 Pro.
The Switch with a Tegra K2 would basically be a Switch Pro, and it's not really a game changer. The only significant difference maybe would be the handheld being dedicated 1080p output and not 720p making less need for OC when the device is docked.