Não realmente. Há uma pequena diferença de velocidade, mas a adição de RAM sempre o tornará mais rápido, mesmo que não seja compatível.
Fonte: Meu laptop e minha área de trabalho tinham conjuntos de RAM não correspondentes.
EDITAR: De acordo com o link (aplicável também ao PC)
Dual Channel: many Apple machines use dual channel memory. In a few cases, this means that memory must be installed in matched pairs. Your computer's manual would say so if that's true. For the majority of cases, unpaired chips will work fine, but matched pairs will result in a slight speed increase (on the order of a few percent).
Depois, sobre o assunto de memória mais barato:
When buying memory, it is generally not advisable to go with the absolute cheapest stuff you can find. Low quality memory can and often will cause application crashes, total machine crashes, and data corruption. Do your homework on quality reputation up front, and be advised that while buying generic memory saves you money, you assume a greater risk of trouble later on.