「これでもか」 is an expression used rather heavily in mostly informal speech.
I would suggest that you think of it as an embedded question within a longer sentence. The 「か」 is indeed a question marker. This should also explain why the quotative particle 「と」 or 「って」 will always follow.
「これでも」, by itself, means "even (with) this (amount/degree)."
So, what is the question implied by 「これでもか」? It should generally be along the lines of:
"Isn't this enough?"
"Aren't you satisfied with this amount/degree/frequency?"
"Wouldn't you call this an onslaught?"
Thus, by adding 「ってほど」、「ってぐらいに」, 「というほど」, etc. to 「これでもか」, you are essentially saying:
"as if to say 'Isn't this enough?'"
That is why I had to mention the quotative particles that go with the expression in question. The heavy use of embedded questions is a feature of our language, which is why we use 「という」、「っていう」、「っつう」、「っちゅう」, etc. all day everyday. We love to quote things without having Japanese-learners even notice it. To prove it, SE is full of questions about this.
こぼれいくらがたまらない！これでもかって程の大粒いくら丼！
describes the surprisingly large amount of salmon roe poured on the rice. "as if to say 'Aren't you satisfied with this amount of salmon roe?'"
ウルトラマンFE3 これでもかというぐらいに撃つ
expresses the intensity of his attack. "as if to say 'Isn't this enough shooting?'"
これでもかってくらいのドロップキック！
describes the strength or the unexpectedly large number of the kicks. Which one it refers to, we do not know without further context.
