The question is repeatedly asked whether '..modification X will void my warranty?' Void your warranty on what, I ask. The fact is that there is no magic component which, when installed will 'void your entire factory warranty'.
This fact cannot (apparently) be emphasized strongly enough. Your dealership can spin it every which way, but the law is quite clear on this issue.
Fitting an aftermarket sky-hook or heavy-weight to your WRX can only initially result in denial of coverage for the OEM sky-hook and heavy-weight. However, if your dealership can demonstrate that your modification caused some failure of another OEM component, they may refuse coverage on that OEM component.
The Magnusson-Moss act commentary is available here: