I also wear a Tilley size 8. I've also raced wildwater C2 for about
10 years and need a helmet that does not squeeze me.

The only, and I mean the only, really big helmet I've found is the
Rush by Grateful Heads. See http://www.gratefulheads.com/rush_lg.htm

My brother has a Drop Zone - it is probably an XL for say up to a
24.5" head, but for the 25"+ size like me it is too small. The Rush
fits pretty well.

I got the next to thinnest foam insert and shaved it a little to make
it comfortable.

A little funky (my wife does not like it) but it is truly comfortable
to wear.