If bandwidth over the existing link is too saturated, then you need to make that tough decision regarding whether or not you should try to squeak out more out of the bandwidth you have via acceleration vs. adding more bandwidth. If you step back and really think about your transport needs, you may determine that you don't necessarily need an OC3 from a Tier-1 provider at both ends of the link.
Realistically, you can get tremendous value and lots of bandwidth from a business class cable modem at both ends of the link. By building a VPN and routing your replication traffic over the cheaper bandwidth, you're not only taking the burden off your primary link, but you're now able to replicate storage traffic faster and in real time if need be.
With top tier solutions easily running into the five figure range, it makes good sense to avoid falling in love with WAN acceleration at first sight, especially when it comes to off-site storage acceleration. If bandwidth between site A and B are prohibitively expensive, or you need lighting fast application acceleration at the same time, then all bets are off and you should shop for the right appliance. However, for simple storage replication alone, why not consider other options?