I just (around Thanksgiving) brewed a Belgian Imperial IPA using Zythos and Citra. It turned out fairly good, but if I think it was under-hopped. I went for mellow bitterness (accomplished) and lots of flavor/ aroma (under-accomplished). I tell you this as it may influence your hopping rate. Here is my hop bill/schedule:
2 oz. Citra whole leaf in the mash
1 oz. Zythos First Wort
1 oz. Citra 15 min.
1 oz. zythos 15 min.
3 oz. Citra 5 min.
2 oz. Zythos 5 min.
2 oz. Citra dry hopped for 7 days
I thought I would have a major hop bomb. Maybe it was the Imperial part (10.5 ABV) that changed the hop impression, but the alcohol is integrated, so I don't know...
If I wanted to up the hop presence on this recipe I'd move the 5 minute additions to flameout and do a long hop stand. I'd also double the dry hops.
Also, if you aren't already at the upper limit of your mash pH, you can try bumping it up 0.1 points. There have been reports that low pH can mask hop expression.