Personally, I wouldn’t leave out the “is” either, but according to the sources I’ve seen (Focloir.ie, Potafocal, Graiméar Gaeilge na mBráithre Críostaí) it is optional, at least in Standard Irish.
In Graiméar Gaeilge na mB. Críostaí there is only one example of “cé chomh” – really without “agus”.
There are two further examples of “cad é chomh” – with agus.
In foclóir.ie there are lots of examples (more than 50). But only two without “agus” or “le”.
In potafocal.com there are perhaps hundreds of examples – an overwhelming majority with “agus”.
(On the first 10 Google search result pages I found only two results without “agus”: https://www.google.com/search?q=”cé+chomh”+site:potafocal.com&safe=off&client=opera&channel=suggest&ei=zTDXUczYBcbYPZyggfgJ&start=90&sa=N&biw=1366&bih=604)
I would think these are just mistakes.
On the other hand you can say “cá mhinice atá …” and “cá mhinice agus atá …”, too.
This “agus” is really optional (at least there are dialects with “agus” and others without it, afaik).
“Agus” here is obviously an intrusion in analogy to the alternative version “cé chomh minic agus atá …”.