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I'd expect the performance hit from querying over a dblink would be far more than from an uncached ordered sequence. I'd suggest benchmarking with an ordered, uncached sequence and seeing if your predicted workload level sees contention at all. My colleague Christo Kutrovsky did some testing on this a few years ago, and blogged about it: http://www.pythian.com/news/383/sequences-in-oracle-10g-rac/
Using an ordered nocache sequence with RAC will likely be extremely painful - it will prevent scalability because it forces a serialization point. You'd be much better just limiting this application to a single node if a single node can handle the workload.
BTW, even using ordered nocache sequence w/o RAC is a scalability hurdle, just gets exacerbated with RAC.