are the php.ini default settings ok to go with?
They are... but they always depend on what you're hosting. WP for example, most have very limited files to render styles and such, so that would be overkill... yet like Xon outlined, for XF... probably best to raise it.
Here's what I have learnt about reading around about optimisation settings: don't believe anything that is given, and instead tweak one setting at a time and measure for the results.
Some say turn down x setting, others say increase it. Increasing some settings, whilst it sounds like more is better, actually causes performance degradation upon measuring the outcome.
Adjust, measure for 24hrs. If you see improvement, keep tweaking. Often best to try both upper and lower settings to see effect, yet some are pretty standard for their setting direction.
MySQL settings for example, I ended up reverting mine to default because some settings whilst they helped, after talking to a DB guru, they told me to try removing a setting and see what happens. It actually improved performance... so I let
@MattW go through it instead and I may tweak it in time myself.
Trial and error is the simple answer.