will a mtb medium cage mech work on a 11-27 ultegra cassette
building a cx bike for the wife
flat bars
mtb levers & mech's
my old cx wheelset
A medium works on my 11-36 with a double chainring, so I would say yes it would.
What chainrings?
It probably will though, certainly does on my cx with 11-28 and 50/34
http://bicycles.stackexchange.com/questions/7264/how-to-calculate-the-capacity-of-a-rear-derailleur
Medium cage capacity is 33. 27-11=16. 33-16=17
So, as long as there's not more than 17t difference between the chainrings, you're fine
i was going to put a road triple on there 24/34/46
That's 5 teeth over capacity. It may well still work so long as you don't use silly gears like granny ring and small sprockets.
Besides, a road triple is usually more like 30/40/50. That sounds more like an mtb triple in the touring flavour which may cause chainline issues with road bike hubs