I have the Ness Big Sucker stage 1. I'm starting to see some oil seapage on one of the ears that attach to the heads near the breather bolts. I'm also seeing some more than usuall oil in the filter so I'm thinking the design is off slightly or the o-rings have shifted slightly. I've had it for 1.5 yrs now. I may change to K&N, V&H or SE and use the stock oval cover.
I can tell you one thing, I put the Ness AC on before my pipes (had the TFI on already) and when I rolled on the throttle the sucking sound of the AC was awesome. Once I put pipes on, I couldn't hear anything except the pipes!!
Good luck!