Extending the mainsail - good or bad idea?

[H35]

We had our boom brake off the other day. No disaster, it was in a bad condition. Anyways, the rest of the rig was updated/switched to a new and stronger a decade ago. The boat (and boom) is 41 years old.
Now is a Golden Opportunity to get a bigger and better(?) mainsail. My thoughts are on making the foot of the main longer, and also have a fully battened sail with a wider leach/«cape»(??).
The question is, how far can I stretch it before the boat turns difficult to handle? Today it’s completly neutral on the helm, no matter wind or which headsail we’re using. So my idea is that I can at least have some more mainsail without ruining the beautifully balanced stearing?...

The foot is 3.38m today. Could I possibly stretch this to 4 meters? Or am I totally wrong...

 

[Tranona]

The danger in increasing sail area aft is introducing weather helm. The designer will have done his sums to achieve a balanced boat which by your account it is. So you need somebody knowledgeable (perhaps a good sailmaker?) to do the sums again with a longer foot for the main. The sailplan looks like you already have a lot of roach in the main so any extra area would be low down and not sure what you would gain from that. Increasing sail area higher up where it does more good would be constrained by clearance to the backstay.

Suspect you would get the best out of the boat by investing in a fully battened mainsail and better quality fabric.

 

[wilkinsonsails]

A more modest increase would be better .The new leech angle and roach would possibly touch the back stay unless it's a split one .