The airbags didn't save a of weight, I wasn't there when the shop removed it, I'd guess less than 5 pounds.

For the coolant stuff, I did that with Audis before, but the Jag coolants if I recall correctly states that you can't add water to the coolant so I never put water with the coolants.

 

Going from memory here but I am sure either or both the Owner's Handbook and Workshop Manual state that the stock coolant is a mix of 1/3 coolant concentrate and 2/3 demineralised water (maybe 1/2 and 1/2), so for a small top-up there is zero problem using plain (but demineralised) water.

 

How about maybe dropping in half a bottle of water wetter, not tried it myself on the Jaguar but I found it worked very well on my Mustang, but it worked the best on my BMW M3 as it been a track car does mean that track driving pushing the stock cooling system right to the limit, adding 1/2 bottle water wetter kept the coolant temperature on track 5-8c cooler than without.

 

Have you already done your research on seat brackets and sliders for the Recaros?