Some of the links on this forum allow SMF, at no cost to you, to earn a small commission when you click through and make a purchase. Let me know if you have any questions about this.
Not 100% sure, but I have heard of several people letting the temps get away from them and it running above 300°. As for low temp... I imagine you could run as low as you want. Just start 3 or 4 pieces of lump/briquets, should give you a very, very low chamber temp.
Peak temps will probably depend some on type of charcoal, I think lump will give you a higher temp.
I have done rib roasts where I held 375 to 425 the whole time on my WSM using lump.
There are some tricks regarding setting the lid slightly ajar to get higher temps. What I did was set a piece of 6 gauge copper wire between the lid and midsection. This increases the draft and therefore the fire and temp.