I have had good experiences with lump in the WSM. Some people have reported temperature spikes at time, but I have not. For me, the transition was a breeze. As fary as using as much lump as briquettes, it depends on the quality of the lump. Royal Oak, available at Walmart, is not very dense. As such, a full ring of briquettes lasts longer than a full ring of lump. I have started using Picnic, B&B, and another locally available lump that are more dense than RO. I prefer that over the briquettes. Now, a bit more care is needed to pack the charcoal ring with lump, due to the irregular shapes. I usually add some, and spin the ring back-and-forth several times to get the pieces to settle, add more and repeat until full.
Good luck with the cook, and let us know how it went.
Rick