Adding more RAM at different speeds? Will it impact performance as much as to make it worse than without adding it?

If your laptop currently accesses the memory at its maximum speed (800MHz for default for DDR3 PC3-12800) and you add additional memory at a slower maximum speed then:

1. Both SoDIMMs will run at the least common dominator.
2. Benchmarked speed for RAM access will drop
3. Actual OS performance will increase as long as your OS can access all the memory. (E.g. no 32 bit windows and more than 4GB RAM).

If the RAM you buy will be compatible, your laptop will run considerably faster.

I bought my laptop with initially 4GB of RAM 1600 MHz, and added 4 more (same frequency). Those 4GB make a big difference when running several applications. Even with the lower frequency the speed will be increased considerably.

The frequency does not make such a big difference.