Why would you write a plug-in for Acrobat X/XI using the Acrobat 8 SDK? If you want to target those two versions use the Acrobat X SDK.
It is not a really big deal, if you must, to have multiple builds of a plug-in with different SDKs. With some ingenuity you can have conditional compilation to overcome any version differences (there will not be many).
If you are prepared to devote the time you can probably adapt the SDK to produce plug-ins which run in Acrobat 8 to 11, but it will require some study and research. A key problem is the Acrobat 8 SDK may include ADM, which is removed in later versions EVEN IF YOU DON'T USE ADM.