If you're happy with the no.5 housing, you could always get your unit 'bored' and the 18t compressor wheel inserted. This I think is what Abbott's offer on their site. Then its a TD04 18t hybrid turbo.
I had this done in my old car as a 'half way house' - it does make a difference. The induction noise at 3k revs as the larger compressor sucked the air in was great. Still makes my spine tingle as I think about the noise. Turbodevelopments did mine circa £280 + vat. I'm sure they could source a N0.6 housing as well www.turbodevelopments.co.uk I think.
Maptun sell the 18t with no.6 housing on their site - but the cost is enormous.