If the meets aren't generating enough numbers then I'd suggest teaming up with other similar groups. You might entice some ex-Tezzaworld people back again.
As for having signed up every Altezza driver, I very much doubt it. I'm seeing more and more of them on the roads, but I can't recall the last time I've seen a Tezzaworld sticker on any of them. I'm seeing 2-3 a day, sometimes up to 6 and they're not all the same.
I think the sticker could be linked to some sort of reward, I believe someone mentioned a private area before? You could hand over a few quid and get private membership access and the sticker in return. Exclusivity breeds desire!
Tezzaworld doesn't appear on the first page in straightforward Google searches like "Altezza Ireland" or "Altezza Owners Club." Dan, you can re-jig the results!
But seriously, not having "Altezza" in the domain doesn't help search results. There's actually very little mention of Altezza on the home page either. I'm not privy to the way Google indexes pages, but I would imagine these factors wouldn't help. I know I found Tezzaworld only after I posted to the Lexus owners club site. People aren't likely to come across the site by accident then, so you are relying on word of mouth.
Last of all, there's a great big stinking recession on right now. Car sales in general have all but disappeared and nobody has any cash to spend on improvements. With that goes enthusiasm, so you are going to see things get quieter for the foreseeable future.