I keep mine at 10k as that's more than sufficient (I now have Amex Plat so I use my BAPP just for booking BA flights and any other transactions where I'll get an Avios boost).

Amex were very willing to boost the limit so I could cover the cost of a very expensive trip to Cuba. The charge was on the card for about a week - I paid for the holiday to get points and section 75 cover and then cleared the balance straight away.