Cable extensions are not the ideal solution since you are adding two connection points to each cable extended. Each connection point is where some current is lost and heat is created. They can be used (and are used often I suspect) but again, not the ideal solution. If you do decide to use them, make sure the connectors are as tightly connected as possible to minimize loss, I'll also suggest checking them periodically to ensure they are not getting too hot, you should be able to hold the connector (while power is on) without discomfort - if used for PCIe, check extension connectors while graphics is working