I'm gonna comment too
There is a very important thing that more weight does, and that is it makes the car easier to drive. Especially on bumpy astro tracks, and in the UK that's all they got in the national series. There's no use for the super fast laptime if you crash twice in 5 mins. Anyone who has done a national in the UK knows these guys have 2 times 3 minutes of practise before qualifying starts. So you better get the car safe and easy, as the tracks they run on are VERY difficult. Of course the top guys also have to be super fast but I bet that even they make a trade off between fast laptimes and a car that is still driveable.
Then there's always the personal feel you want for your car. Some people just don't like how a heavy car feels (like richard for example). Ellis does, but his driving style simply asks for heavier cars. I don't think Tom God runs his car at 1700-1800gr? And he won this year.
Hitting bumps can indeed be solved with changing pistons etc, but one thing you don't change: weight of your wheels+tyres. So going extremely heavy or extremely light will not be good as this messes up the ratio between chassis weight and wheel weight. Only way to test that is by trial and error.
Personally I also don't like super heavy cars, but I also never succeeded in running with very light cars. So I go for the medium solution (as with any setup change I make
). Smooth high grip I will use around 1550-1600gr (2wd) and on lower grip, or bumpier astro, more like 1650 - 1670 max. Depending on grip level, weight will also be moved fwd or back.
As tony says, just try it for yourself, and if you get lost, do what the top guys do and start over again. Or even better, the subtop, as they will most likely have setups that suit the majority of the drivers. I've driven with some cars or seen people trying to drive cars from the top drivers, and usually they can't complete a single lap without crashing out twice in every corner