You should update your version of SimEvents to the latest release. SimEvents 4.0 (released Sept. 2011) better supports large models than previous releases, because of it's enhanced semantics. Model Reference support is not yet available though. Also, in terms of performance for SimEvents, you need to worry about more than just the number of blocks, but also the number of events, since both affect performance. In any case, the newer versions of SimEvents should better support you.