As with most sorbets, be careful not to over-churn the sorbet. Rather than letting it over-churn, rely on the freezer to put the finishing stabilising touch to the sorbet.
The (optional) whipped egg white will improve the consistency of the sorbet, but will matter more in the cases where you do not plan to enjoy the sorbet relatively soon after churning (in other words, no need for egg whites if you intend to serve the sorbet soon after preparing it!)
The Agave nectar (or similar inverted sugar) is useful for improving the consistency of the sorbet without adding too much sweetness. If you do not use inverted sugar, be prepared to add more sugar (and then, suggestively, even more lemon juice).
You can also prepare this recipe in the traditional sorbet way, using a simple sugar syrup, mixed with some Cola soda, and lemon juice. You should use the Egg test to get the sweetness level right, but a good starting point could likely be one part Cola soda per two parts simple sugar syrup (and the simple sugar syrup could be made up of equal parts of sugar and water).