What would be the scientific impact of locating the Thirty Meter Telescope at its alternate site?
The Canary Islands have slightly poorer seeing on average, but the main difference would be the higher column density of water vapour. Mauna Kea is a  higher and "drier" site, which means you can observe into the mid-IR.

Looking at the first light instrument package I would guess that WFOS (wide field imaging at 0.3-1 micron) would not be badly affected. But there would be some impact on IRIS (near-IR spectroscopy) towards the long-wavelength end of its range. 

The near-IR adaptive optics would also be somewhat affected by the slightly poorer natural seeing, but I'm not sure I can quantify this.