You will notice nearly a 5% increase in mileage figures, since the compression increases by a great margin and therefore enhances ur throttle response. Which will proportionally drop the throttle input required for a given speed and hence saving more fuel. The low-end torque on the CBZ will increase a lot and on the karizma (it already has good low-end), you will get a stronger mid-range as well.
The character of the engines are bound to change by a lot.
When an engine is tuned, the volumetric efficiency increases and will actually enhance the overall efficiency, which is both power and mileage.
Some facts from my engines and we have enough proof to prove it.
My 25+ bhp karizma used to give us 33kmpl on highways under speeds of 110-120kph constantly. We have a few bhpians as proof.
My R15 gives me mileage of 30kmpl in street during tests (under mostly constant speed bursts) and on highways around 42kmpl, which is better than what a stock engine gave us. I'm talking about an engine which makes a lot lot more power than stock and is a 165cc engine.

Overboring an engine within safe limits is the best way to make good power with good rideability and improved fuel economy. However the methods have to be very scientific and not some plug and play and burn.

Costs of the kits can be discussed through mails. The kit will be sold with a block, semi-forged custom piston, rings, pin, new head gasket. However it has to be specified if its for the CBZ or karizma as the block differs.

I have been using big bore in my zma since one and a half year without any problems. Just go for it man.

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Bike's performance has been changed Completely. I am in love again. Feels like as if I am riding 250 CC bike. Tremendous amount of raw power. One has to ride it to feel it.