I think you misread the PC cost.

It's $1600.
or $3200 for 6 Mh/s
Or $500 for an additional 3 Mh/s

In other words even with the 13% lost profits from the additional power requirements 2 PC units would bring in about 40% more profit for a 15% base cost increase.
Not a good buy, and that's before considering non mining resale value.