What you're doing is very good. You have a PSU that at MAX may pull up to 6.3 A on the 240 VAC supply, and you are using a cord capable of more than that. The original cord, besides having the wrong plug on the end, was not actually suited for over 5 A although it is likely that MOST of the the your PSU will not use the full 6.3 A. You will not cause any problems doing this.
^ +1 The higher Amperage rated power cord is higher quality part.
The 5A cord might have gotten warm during a full power gaming session where the 10A cord would have been cooler because of the thicker gauge wire used.