Datasheet or termal resistance for this heatsink
Going to Farnell's heatsink search page and entering your dimensions and "TO220" as search parameters, I can't find the exact model, but there are bunches with thermal resistances in the order of 20-25C/Watt. Yours is a reasonable quality aluminium extrusion so better than some; I would use 20C/W as a reasonable figure unless you can find the actual data.

Which means at 7.5W from yesterday's question ... 150C expected temperature rise...

The other option is airflow, of course... 

Theoretical approach: you could re-run those numbers (get a datasheet from a proper heatsink!) and see what airflow would make it work, and if a small fan (another datasheet) could push air that fast.

Experimental approach : attach a thermocouple (I got one with a £10/$15 multimeter - no longer sold but it came from Maplin) to the heatsink, and measure the temperature as you draw more current. (Start at 0.25A on one reg, given your other question. Point a CPU fan at it, add ducts, re-measure temperature. Repeat until satisfactory.