Hi-macs are made by LG-Chemicals, a Korean company that...well...basically own Korea...and Corian is made by Dupont who own a lot of the rest of the world but definitely don't have anything to do with LG-Chemicals (unless they are hiding the fact).
As for seamlessly joining Caesarstone, if you have seen a bad join it is beacuse it was done by a ** installer who was possibly working with a circular saw and a tube of building adhesive.
In this case it is up to the installer to make good on the work, otherwise if Caeserstone find out they will discuss the issue with the installer.
Unfortunately Caesarstone can not specify who is and isn't allowed to install their product; so, as with a lot of other things, you get what you pay for.
If you get your Caesarstone installed by a skilled and reputable mason then then any join should be virtually undetectable - whether the top is light or dark.
Long benchtops are a particularly good place to use a reconstituted stone product where a seamless finish is desirable.
Should you have doubts about getting the quality you want in the joining of Caesarstone just ring them directly and ask them to give you a list of referred installers.
And for anyone in Brisbane, if you have a chance, check out the Blanco showroom if you want to see some well installed Caesarstone.
Cheers,
Earl