This is the reply I got from SOA when I asked them if installing an SPT exhaust myself would affect my warranty
Thank you for taking the time to contact us. I hope that you are enjoying your 2011 WRX, XXX.
Installing genuine Subaru parts designed and offered for your 2011 WRX will not void your warranty. Some SPT parts are sold "as is" without any warranty coverage and others only have a limited parts warranty.
The SPT exhaust, part number D441SFG400, is new for the 2011 WRX 5-door models. This is the only SPT exhaust that we would recommend for your vehicle. If you were to install an SPT exhaust not designed for your specific WRX and any problems arise, the warranties on your vehicle would not cover the repairs. The same would be if you were to install the SPT exhaust yourself and any problems arose caused by improper installation.
Thank you for the opportunity to be of service. If you need any future assistance, please let us know. Have a great weekend, xxx.
John J. Mergen
Subaru of America, Inc.
Customer/Dealer Services Department