The experience of Jerry (his SO has a Vanagon he installed them on) is
that you will notice the difference at cruising speed due to better
breathing at high rpm.  He also noticed much better climbing ability.
We have a mountain pass here on I-90 where you climb about 800 feet in a
couple of "straight line" miles, and he is able to maintain 70 mph with
the mod, where he had to downshift to third before.

One person's experience anyway.  I wouldn't expect any mileage increase
however, when you get a performance improvement you tend to use it and
usually decrease your mileage!
--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia
'65 MGB (Driven since 1969)
'74 MGB GT (Restoring)

Assisting on Restoration:
'72 MGB GT (Daughter's)
'64 MGB (Son's)

Parts cars:
'68 & '73 MGB, '67 MGB GT