Induction kit = Warranty straight out of the window, afraid.
As for gains, arguable, provided it gets loads of cold air feed it should pick up something, don't know if it affects the newer cars so much but some of the classic shape cars really hated induction kits and had a bad habit of chucking their MAF sensors due to the oil from the filter coating the wire and burning it out.
Loads of people stock them -
AWD
H1Performance
DASTEK
Star performance
Falkland
etc
etc
etc
Really depends on how far you want to travel and who you want to supply and fit it for you.
John.