What is the impact on the Cost-Based Optimizer (CBO) when implementing ISCSI SAN solid-state drives
The cost-based optimizer does not (currently) change its estimated IO costs based on your hardware.

You say this question is geared towards answering what filegroup and partitioning strategy is best for your environment. In that case, who cares what the optimizer does? The question is what works well in your environment, and the real answer is to test it.