When GPU mining was introduced about a year ago it was only a fraction as efficient as it is today. So there has been much improvement in the months since. However if there are any opportunities left to speed things up, they are likely only marginal improvements.
With open source, improvements from one miner release generally propagate into other releases. So even though the total network’s THash/sec can increase thanks to improvements, a miner’s share of the pie remains the same.