Hi Simon,
That remains one of the issues of multi-class racing, the larger engined cars generally have the ability to produce more power, offsetting their often heavier mass and in many cases exceeding the mass penalty. As you will have encountered it will often make the slower big class cars fast down the straights but slower than the lower powered cars (such as the Suds) round the corners! When I raced a Sud back in the mid 90's I used to have huge battles/frustrations with the 3.0 V6 75's and 164's - what I would lose in a straight line I could make up under braking/cornering. Unfortunately if there was a straight before the finish line they would blitz past again and beat me. I had to console myself that they weren't in my class - but beating them brought greater satisfaction. The game is up when you have a much higher powered car that also handles well!
Ultimately, one has to be quicker through the corners that precede the straights so that they are only one your tail as you enter the braking zone - so you can hold them off and stay in front. Playing to the strengths of what you have really - and some circuits will play to this. You will find that if you stay clear of them then you will pull away in relation to a car you perhaps felt you had no chance of beating due to it's power advantage. The percentage deficit to Andrew Howards Jaguar at Silverstone will be much higher than it will at Cadwell, etc. The most entertaining races that you guys have excelled in this year have been in the wet where the conditions have negated their power advantage.
I have similar up against the 1100 kg V6's and 2.0 turbos - some with over 400bhp against my 240bhp and 780kg - despite the lighter weight I'm carrying a disadvantage which shows more at Silverstone and Oulton than it does at Cadwell or Croft.
I think it should be said that you have both just completed your first seasons with cars and circuits that you are learning the characteristics of - and you have both done really well

You will most likely find that you will instantly be quicker next season at repeat races - even though you may feel no obvious improvement in your driving, it will just be a confidence thing! Any improvements to the cars will only help reduce your times. From your post above it already sounds like you are analyzing your season - I always watch previous races back and also others footage - just to see what could have been better. I'd also recommend the AIM Solo GPS timer/logger so you can better understand what made certain laps better than others - with tyre degradation/track conditions its not always apparent. Any advantage worth a time saving is worth it - especially the free stuff like walking/cycling the track to start with and pre-race YouTube sessions!
My personal winter goal is a weight saving from me - the aim is to lose 1kg a month for the cycling event I do in the Alps - but the saving will benefit the motorsport too!