I had a single piece ferrule/tip put on a while back and it didn't work out. It actually broke twice. The first time, one side of the phenolic cracked. The cue repairman said it must've been a bad piece of phenolic so he replaced it free. The second time, the whole thing split down a side. He said it may have something to do with coring out the phenolic ferrule to fit on the shaft tenon and getting air pockets between the top of the shaft and the ferrule, which cause it to "explode" from impact. I'm not sure if this analysis was correct, but basically, the one piece was a piece of crap.
BUT, we came up with a nice fix. First, he put on a hollow phenolic ferrule. Then he turned the phenolic down on a lathe until it was a little narrower than the shaft. He put an ivorine sleeve over the phenolic ferrule (to make it look like a real ferrule) and matched it up to the shaft diameter. At this point there were 3 layers: ferrule (outer sleeve), phenolic (middle), shaft (inner). Then he topped the whole thing with a 3/8" thick piece of phenolic. After shaping it, it hit like a ton of bricks. I haven't had a problem with it ever.
I know it sounds like a lot of work - and the extra layer isn't really necessary, but I didn't like the look of a brown phenolic ferrule. It looks like a standard ferrule now. I'm sure if you simply topped a phenolic ferrule with a phenolic tip you'll have great results.
-td