You have the wrong weight for Excite (Spike). The weight is actually 15.74g - see the H&N website here
https://www.hn-sport.de/en/action-shooting/spike-22
This suggests your power reading will actually be quite a bit higher, given the pellet is heavier than you thought.
The Mk3 is quite pellet fussy, given it is an old .22, ie 5.6mm whereas the German calibre is 5.5mm in terms of their barrel size. The Mk3 therefore requires a slightly larger pellet. I have found that BSA Elites do well with these but so, too, do Superdomes.
You should be able to get over 10ft/lbs with the right pellet. Mine does between 10.1-10.5 ft/lbs, which is what they were designed to do.
I might mention that I have found Excite (Spike) extremely accurate and consistent. It is one of my favourite pellets.