As stated above (and probably before your post came through), the Hiface has worked for me.
The only way you will know if an asynch USB will sound better is to buy it and try it. Also, you should be aware that Steve N. of Empirical Audio (see post above) sells the Off-Ramp, an asynch protocol USB converter for $799. This would be in addition to the DAC you already have. The Hiface sells for $200. Only you can tell if the difference is worth it.
The only way you will know if an asynch USB will sound better is to buy it and try it. Also, you should be aware that Steve N. of Empirical Audio (see post above) sells the Off-Ramp, an asynch protocol USB converter for $799. This would be in addition to the DAC you already have. The Hiface sells for $200. Only you can tell if the difference is worth it.