I will probably get called out for this, but a long time ago when I had my first rev d GTB I remember reading Gulls own information about their 98 blend as I was about to fill up for the first time at a Gull
Their brochure actually implicitly recommended to NOT use Gull 98 in our cars (I am guessing because of the ethanol? Or some additive?)
Try a tank of BP 98. As for the sluggishness you mention, does not sound right at all. Even an auto rev d should give you a decent pull from a stop if you give it the beans.
Take a vid?