Thanks for your reply.
Would the S5 not be better value than the S3? Yes it's 4 times cheaper, but it's also 4 times less powerful etc.
Interesting idea with the solo/regular pools. I'll probably do something like that.
Regarding profit, I worked out that running an S5 at $0.08 - $0.10 / kWh would be easily profitable...
Also, does anyone know anything about the S7? If it's something that could end up costing $1000+ I probably wouldn't be interested.