Upgrade to dura ace mechs?

[Ming the Merciless]

Get some dura ace valve caps. I did and instant 3mph boost to speed.

 

[wafter]

What would you upgrade then and don't forget it would have to be 'invisible'

That would depend on what required upgrading

Sounds like the bike's fine as it is so in your position I'd chuck the money in a slow-burn pot for future essentials / save for something you actually want or need, rather than going out of your way to spend on something.

While I'm happy to spend within my means to make my bikes as I like them / fit correctly etc, I'm yet to see any value (for money) in many of the common upgrades. For example I worked out a while ago that through the most efficient mass-saving medium of wheels, each improvement in acceleration of 1% would cost me something like £400, while I could get the same effect by binning carbs for a week or two

 

[Blue Hills]

Don't know if you do your own maintenance.
If not spend some money on quality tools and associated stuff to keep the bike running smooth.
I find bike fettling quite therapeutic - and can feel the difference in the bikes afterwards.