Along with what rkoaudio said regarding comfort level and skill, one nice thing about letting Ascend do the upgrade is that your 7-year warranty is renewed. Plus, you can get printed measurements of the speaker's frequency response.
The components themselves are built within tight tolerances. IMO, there should be no audible differences between a DIY Sierra-2 pair and a factory-assembled Sierra-2 pair. Dave's been down this road before with the NrT upgrade.