Peter,
you do almost have to be at full throttle to achieve kickdown, same on every car I've driven with a 727 Torquelfite.
However it definitely shouldn't still be in third/top gear when you're coming out of a roundabout. Sounds like your tranny isn't set up properly, or perhaps your fluid level isn't right, that can cause odd behaviour (needs to be checked with tranny warm, with engine running, in Neutral).
There are lots of different modification kits for the 727, but I would get it working properly before you contemplate fitting one.
Having it kick down earlier may not be that desirable, especially going round a bend in the wet
