Is forcing GPT-5.6 Sol on every prompt actually better than letting it route to 5.5 Instant?

Where the higher tier earns its latency, from a fortnight of doing exactly this comparison on my own work:

• Multi step reasoning where an intermediate result feeds the next step. This is the clearest win, and the failure mode of the cheap tier is confident and wrong rather than hesitant.
• Long context with contradictions in it. Two versions of a spec, or a thread where the requirement changed halfway. Low effort tends to answer from whichever version it read last.
• Anything where the first plausible answer is wrong. Debugging with a misleading error message is the canonical one.

Where it is just slower: retrieval shaped questions, formatting, short self contained snippets, rewriting. If the answer is essentially lookup, effort buys you nothing.

The cheap way to settle it for your workload: keep ten prompts from real work, run each at both settings, save the outputs. Twenty minutes, and it is your workload rather than someone's benchmark table.

Do not pin it globally. Pin it where the work is.

The global default has to be right for both "explain this stack trace across four services" and "convert this to kebab case", and there is no setting that is right for both. Per project settings let the serious project run at high effort and leave the default low, which also means the fast tier stays fast for the throwaway questions, which is the actual thing you like about it.

Take your own observation seriously: you said you cannot tell the answers apart on half your prompts. That is not a failure to perceive quality, that is a measurement, and it says the cheap tier is the correct choice for that half.

The useful version of this question is not "which model is better", it is "on which of my prompts can I detect a difference blind". Everything you cannot detect a difference on should run on whatever is fastest. That is a decision you can make today without waiting for anyone's leaderboard.

One trap when you do this comparison: on eligible paid plans the Instant setting can step up on its own for requests that look demanding. So your "Instant" arm is not guaranteed to be Instant throughout, particularly on exactly the hard prompts where you are trying to show it falls over.

If you want a clean comparison, use prompts that are unambiguously hard and unambiguously easy rather than the interesting middle, or accept that the middle of your results is measuring the router rather than the model.

Latency is a real cost even when it is not billed. I pinned high effort for a week and stopped using it for the small questions entirely, because a four second wait breaks the flow that made the quick questions worth asking. Ended up back on a low default with one high effort project, which is where everyone in this thread seems to land.