The question seems rather moot here- whether or not one thinks the eyewitnesses are somehow more important or not doesn't terribly matter; St. Matthew and the rest did not offer interpretation to the Eucharist (one might say St. John did, but he did not explicitly link the chapter six discourse to the Eucharist). St. Paul does- "The cup of blessing that we bless, is it not a participation in the blood of Christ? The bread that we break, is it not a participation in the body of Christ? Because there is one bread, we who are many are one body, for we all partake of the one bread." I found it hard to get around this- and because of it I began to read "This is my body" without seeing only a symbol.
At any rate I am at a loss to see why the Gospels convey the Real Presence more strongly the Paul's Epistle to the Corinthians. Besides, it is not very catholic to elevate one part of Scripture over another- leave that for the "higher critics" and Jesus Seminar people.
While this may be true, think of when scripture is incensed at Mass. At the Gospel. The Gospels are given to the bishops at their elevation Mass. The Gospels have a different place. And one Evangelist was actually a witness to all that happened.
We may not elevate one part, but the gospels are certainly treated differently.