15 psi is the right range for pouring a hefeweizen at 38F, my chart says that's 2.8 volumes of CO2. You just need to extend the lines.
I carbonate and serve at the same pressure, no worries that way. I just use a longer line for higher carbonation beers.
If you want to speed up carbonation, you can rock the keg to get it to go into solution faster. You can also buy a keg lid from morebeer that has an carbonation stone attached, it's great for carbonating kegs quickly. I don't recommend going with higher pressure to carbonate faster, you usually just end up with overcarbonated beer. The other thing is, if it is pouring too fast you'll lose a lot more CO2 so it may seem flatter than if you had a smoother pour.