I don't think you can get to that weight with WTAC unless you go to pro class.
You are better off with ducting from a undertray than through the window. Roll cage will be in the way of any decent ducting (already looked at this one some time back).
There are a few guys on this forum who have some pretty serious track cars with plenty of info on their builds which would be a great help to you.
Getting 1000kg from a T top SW is a lot of work but possible. If I had my time again, I would start with a hard top shell, bead blasted, trimmed, spot and seam welded, cut the rear arches out then pop on a widebody kit before doing anything else. There are some new roll cage tubing which is lighter than what I used a few years back.
I would also recommend fitting double A link front and rear as it will save you money in the long run and you will have a far better sorted car. Basically, buy the front and rear arms and hubs off a Lotus then fit these to the car. Lot better than fighting the existing system for years on end.
Do you have a budget in mind? For time and money?