As far as end cans are concerned there probably isn't one that makes any significant difference.
If you go for a full system then again, whilst there may be a few HP either way whether or not you would notice the difference is debatable.
That said Mark's Pipes get consistently good press. In my own case the Dale Walker Holeshot's worked well on my bike.
Biggest HP gain would be by getting the OE carbs set up on a dyno in combination with (say) one of Sean's kits.
The air filter in a standard air box is not the restricting factor so changing for a free flow variety would not give any immediate benefits.
The inlet formed by the Y piece on the top of the air-box is the restricting factor. This can be shimmed, drilled or removed. You would still need to get the fueling set up though.
Next option would be a Stage 7 kit but you would also need to budget for a modified breather.
Finally you are into flat-slides, either MIkuni or Kehin but now you are in to significant cost!