Personally I don't see any flaw on doing this. Assuming that currently your SMTP traffic is flowing from the ASA outside-> inside interface -> to the internal-ACE VIP and from there to your servers, if this is the case, then moving your exchange servers to the DMZ will only represent a small NAT/Routing configuration on your ASA and a matter of replicating the configuration from ACE to ACE considering the IP changes.
The other (unlikely) scenario is that SMTP traffic is not going through the ASA and somehow routed directly from the internet facing router to the ACE, in this case you'll be adding security to the connection which is just great!
I feel that most of the considerations need to be made from an ASA point of view (security-levels wise), on the ACE side, no biggie!
I don't see a problem with your new design; just keep in mind you'll be moving to what's called a "routed" mode so the DMZ-ACE needs to be configured with an IP from the internal VLAN, you can either configure the current connection between the ASA and ACE as a trunk or just connect one of the ACE ports to the internal switch.
You'll need source NAT performed on the ACE to avoid asymmetric routing specially for the request coming from the internal VLAN (reverse proxy).