I can't speak for the accuracy in your part of the world, but here in Brisbane, the Navionics cartography is streets ahead of the CMap or Blue chart equivalent. I've been selling and repairing Lowrance and Navionics product for quite a while and have no hesitation in fitting them wherever possible. I also sell Blue chart and CMap but prefer the Navionics where possible. In answer to the other points raised - any current model Lowrance plotter should redraw a full screen chart in under a second - if not there may be a software issue with the plotter. Some early software versions were a bit slow, but updates are available free of charge on the Lowrance website at www.lowrance.com
The MMC Navionics charts cannot be copied onto another MMC. The chart is coded to the serial number of the MMC, and will not duplicate onto an MMC with a different serial number. Generally speaking, all electronic charts are copy protected in this manner. However, a backup can still be kept - just copy/paste the file off the MMC onto PC hard drive to backup a copy. Provided the file is saved back onto the same MMC it was supplied on, the backup will work fine. So if you manage to erase or format the card, all is not lost.
Cheers - Chris