Boxer engine is capable of reaching 9000 without any mechanical problem. But..... You have to have the engine tuned for that purpose which means correct timing , extra fuel feed to the engine and an overall good status. The only problem has to do with timing and fuel. Here in some Tech School have boxer engines that , for testing purposes, rev the engines up to 13000 / 140000 rpm in order to create problem to them in order to fix them.
John , for 10 years was constantly revving his 1.5 engine up to 8000 rpms and more and with over 200.000 klm but his engine was fine tuned and it lasted another 150.000 klm before replacing it with 1.7.
Sprint 1.7 '85