Anecdotal Evidence (no hard data here):
I got 2,000 hours out of the Mags on my old old Grumman - no blast tubes.
I have now got 900+ hours of life (so far) out of the mags on my RV (and the internals looked great at the last inspection) - no blast tubes.
I'm sure that you will get a variety of opinions on this - I'd rather keep the cooling air on the cylinders. People have sealed up very small holes and gotten lower CHT's - why open up a couple more "leaks" in the pressure plenum?
Oh - Louise's RV-6 has blast tubes - 2600 hours - no problems! So maybe all my experience cancels out.....
