This group does cause a lot of confusion.  I'm on my lunch break at
work now, so I don't have a copy of the Compendium in front of me;
but I do recall the illustration you refer to.  If I remember
correctly, the slender shell you mentioned is actually Hemifusus
carinifera, which is not the same as H. ternatana.  Many authors
consider H. ternatana, H. tuba, and H. colosseus to be conspecific.
H. ternatana is a smooth-shouldered, rather lightweight form; H. tuba
is a spiny-shouldered form; and H. colosseus is a large (commonly over
300 mm), heavy, tall-spired form.  All three are rather common in the
waters around Taiwan and Japan.  The ternatana form also occurs in
the Philippines, but I'm not sure if the tuba and colosseus forms
occur there.  H. carinifera is common in the Philippines though.  It
is a more slender species, with heavier spiral sculpture, and a very
long siphon canal; and it is commonly medium to dark brown in color
(all the forms mentioned above are typically a light cream color).
H. carinifera seldom exceeds 125 mm.  I believe (again working from
memory here) it is pictured in Shells of the Philippines.   I believe
H. elongata may be a synonym for H. carinifera, but I'm not certain on
that point.
Paul M.
Rhode Island, U.S.A.