Throughput increase/decrease by how much percent
Improving throughput at the expense of latency

Delay: D = T + nδ
Throughput: IPS = n/(T + nδ)
Choose non-critical paths to merge in series to add latency but less than the critical path.

Now compute and see your critical path of 400 is not exceeded with 2 pipe merged in series, so the throughput is not changed, where latency δ can be different for each pipe rather than equal as shown above.