Well i had all six out today in about an hour total.
Super easy job, M5 tap was an absolute waste of time. Dremel, little tickle with the blow torch, self-tapping screw and a slide hammer......out they come
And the results are......... 6.24mm the valve
And the hole...6.285mm .......ish
Unless you're an animal there is no way you should damage the hole and require oversize valves!!