I played around something similar with:
* AMEX Explorer
* Taobao World Mastercard (BOCHK)
* Chill Card (BOCHK)
* Visa Signature (HSBC)
I tried to do the same value transactions in INR, within a few minutes to max an hour of each other. Here were my findings:
* BoCHK Taobao: +0.15% of midmarket rate (xe.com)
* AMEX: +1.5% of Taobao
* BoCHK Chill: +1.92% of Taobao
* HSBC Visa Signature: +2.06% of Taobao
I didn't try the HSBC Debit Card even though I have it unfortunately, but maybe it would've been cheaper than BoCHK Taobao (maybe coz HSBC can also get better INR rates than BoCHK?)
I'm going to the UK next month, will try and buy the same pint on every card I own. Good way to get drunk!