Robert,

I'm kind of confused by your description that you're over-shooting your waypoint.  Can you tell me a little more about by how much you're overshooting your waypoint?  I suspect you must be using the A78 to navigate to a pretty precise degree if you're using a trolling motor.  

As far as processing power between the a78 and the es128 you're looking at a pretty big difference.  The es128 is a full generation newer and a member of a higher end line.  Though Raymarine doesn't specify a ton about their processors there's no doubt the es128 is quite a bit faster.   

Networking your es128 and the a78s comes with some complexities.  The es128 can run Lighthouse II or III while the a78s are limited to only running LH-II.  On my boat I have a pretty large network of LH-II based MFDs and I've observed that when the a series are running on the network I think there's some struggling to keep up and process everything.   

Also, from your description it sounds like maybe you're considering networking the MFDs together without use of an HS5 or similar Ethernet switch.  I'd really recommend you use a switch and avoid any network anomalies resulting from unsupported configs.  

Ben

I am saying I am over shooting by the boat icon on the chart seems not to match the sonar.  I add the external gps thinking it would fix the problem but now I think the A78 can not update the screen fast enough

I am not sure if engine data is slowing things up

Yes I tried direct wire between the 2 mfd ‘s and it seems to work fine

I was thinking to keep the A78 dealing with engine data  and move everything else to ES128 The GPS and mercury gate way on canbus  to A78

Sorry Robert, something is still not clicking for me.  The boat icon's position is set by the MFD's GPS fix, either from the internal GPS receiver or from the external one you've added to the Seatalk NG network.  Sonar isn't fixing your position so trying to match position based on sonar doesn't seem quite right.  

I don't have intimate knowledge of the internals of Raymarine's OS but I believe a large share of the workload is carried by the data master regardless of which MFD is displaying data.  For instance, anything happening on Seatalk NG is only being processed by the data master.  The other units on the network have their Seatalk NG interfaces disabled.  So, if you're going to try and offload you're likely best served to break a device or two off the network and run two separate networks.   

Ben

I am trying to steer the boat via the boat icon on the chart to the  X on the screen which is my waypoint. I never seem to get the two to meet up on the chart