Put a thermocouple or temperature dots on the housing by the mag coil and see what the temp is with, then without the blast tubes. Then you know for sure. Champion Aero says 185F is their limit. You might want to make an allowance for a "hot" day to ensure it is always below 185F.
Bill and I very often think alike. I had phoned the same engineer at Champion and he called back yesterday. 185F is the official line, but he cheerfully agreed that mags see more, often much more, in service.
I have routinely measured breather gas exit temperature at more than 200F in flight. Oil temperature is measured post-cooler, so sump (and accessory case) temperature is higher than indicated. Cowl air exit temperatures are coming in at 125 to 200 plus; air in the accessory case area is also pretty warm. And we've seen P-mags measuring 200+
after shutdown, when blast tubes do nothing.
I don't
know how much a set of blast tubes would affect cooling pressures, simply because I've never been inclined to install and remove a set just to make the measurement. Now if somebody already had a set installed, it would be relatively easy to get pressures, land, tape the tubes closed, and immediately fly again.
Bill is 100% right. If you really want answers, measure it yourself. One good measurement is worth 1000 opinions.
Information about rigging temperature probes and pressure piccolos here:
http://www.vansairforce.com/community/showthread.php?p=977603