Warning: I don't have and never have had a Speccy. (But I have messed about with shocks).
Some shocks have a travel spacer. Alter that and you alter the travel.
But first: Release the air from your shock and fully compress the bike. Check all round to see how much space you have. If you need another 10mm, then look for a bit more (say a total of 15mm) between the tyre and the seat post, or between the top of the seat stay and the seat tube.
Because the shock will be tilting forward a bit more, check that there is space between the shock body and the down tube.
I've been wondering the same thing. You MAY be able to fit 225x75 shock but I have not really measured anything. That would also raise the bike and potentially cause other issues. I'm not aware of any other suitable trunnion sizes.