Are SQL Server Database writes slower with Snapshot Isolation?
is because it has to write to cache and then to the TempDb (row version) and then it can return to the caller.
  


No, this is incorrect. It somehow implies that writes in the presence of versioning have higher latency as each write has to touch the disk (for tempdb) which is not true. The write into the tempdb is also a write into 'cache'. The only 'wait' occurs at COMMIT time when the log has to be hardened. Is true that with versioning both the DB log and the tempdb log have to be hardened, but this does not necessarily implies higher latency (the IO should be paralel on different storage paths, the tempdb is stored on separate drive from your heavily used LDF, right?). For a full blown explanation read How It Works: Bob Dorr's SQL Server I/O Presentation I really hope your DBA understands this better than you are conveying it here.

As I mentioned in your other post: snapshot has no cost for INSERTS and the cost for updates and deletes can easily be mitigated. Row Versioning Resource Usage explains the trade offs. At this moment you should be probably testing with a realistic workload, which is the only way to properly asses the impact you would experience.