Um, no, I'd do it because if I was Jerry Reinsdorf, I would 1) have the money that I'd want to spend to try and be a winner instead of sit on it and stay a mediocre franchise, and 2) Giles and Kendall would make the Sox a better team. So our payroll would be $60-$65M next year instead of slightly over $52M or whatever it is this year -- big wow.
As far as "guaranteeing a playoff run and an increase in ticket sales", there ARE no guarantees in professional sports. But, if the Sox went out and got 2 more all-star caliber players, believe me, the fans would take notice. And if the Sox played up to their standards with the guys they'd have on that team (Colon, Buehrle, Ordonez, Thomas, Konerko, etc, etc), the wins would pile up and so would the ticket sales.
And no, Jamie Navarro has nothing to do with this and it's a bad comparison because Navarro was horrible and Giles and Kendall are very solid major league players.