You don't understand how a 2 tiered bonus works, do you? The 2nd bonus wouldn't be paid until after the 2009 season. So, it would be during the 2010 season and not count in 2009.
"Amounts Treated as Signing Bonuses. For purposes of determining Team Salary under the foregoing, the term “signing bonus” shall include: . . .
In a Player Contract, or any renegotiation or extension of a Player Contract, that is executed in the Final Capped Year, each of the following, if it is to be earned or paid to the player in the Final League Year (which is an Uncapped Year): (a) any Salary advance which the player is not and cannot be obligated to repay; (b) any off-season workout bonus that is contingent upon the player’s participation in less than 32 days of the Club’s off-season workout program;
(c) any off-season roster bonus; and (d) any off-season reporting bonus;"
The system is specifically designed to prevent a scenario where, e.g., a team "defers" $20M as a 2010 roster bonus.
The Patriots are currently 5.7 million under the cap. Even if they signed all their rookies, they'd still be UNDER the cap. In fact, thanks to the rookie minimum, none of their salaries would count against the cap. Only their signing bonus amortizations would count. And their signing bonus amortizations aren't going to eat up 5.7 million in cap space. So, I don't see where you get that the Pats would need an ADDITIONAL 1.5 million in cap space to what they already have.
As I explained in another post, that $1.5M is the difference between the cost of signing all the rookies and the money that would be saved by the players displaced off the top 51 (though it becomes top 53 when the season starts, plus the team needs reserves to deal with injuries).
In other words, just because the Patriots are $5.7M under the cap, that doesn't mean they have $5.7M available to sign Peppers.
One other point, come to think of it—exactly how much is Peppers worth to the Patriots, given that they'll also have to give Carolina draft picks for him? [Who knows, perhaps Belichick was hoping that Carolina would just drop the tag.]