I grew a nice HC carpet with 18w several times on my Mini S. With the 27w you will probably be unlimited in what you can grow, but as mentioned you will need to have a good steady hi-tech setup and rigid maintenance schedule to avoid algae issues.
Interesting; I'm trying to find a balance between cost and wattage, obviously
I'm just afraid how much shipping will be for the Archaea light. It's already quite pricey at $85, and shipping on top of that might bring it over $100. The cheapest eBay 18W fixture I've found is ~$50, and the Catalina 26W light was quoted to be at $67 shipped (and that doesn't include the mounting legs).
They had a sale when I bought them that made it 2 for 1 pricing. They work great and have a 13w pc bulb you can change to whatever color you like. I would personally get their plant growth bulb though.