Some tweaks are tradeoffs , what you gain in one area you lose in another. Overclock the device and you might overstress the hardware and reduce it's lifespan. Use fasterN9 and the interface is smoother while multitasking performance might get worse...and so on.

You can probably find as many preferences for tweaks as there are users on this forum. And thats simply what most tweaks are. One person might want the UI behave differently from the next one.

I for myself have one phone running openmode kernel and one incepted device. If one likes to tweak the basic linux userland and system stuff (partitioning, swap usage, startup processes, shells etc.) stuff by hand, then the openmode is easier and less prone to errors. Much of that tweaking is just for fun of it and rest might be for some functinality/feature that particular user finds useful and wants to enable on the device.

If you want to overclock or add occasional user interface tweak, then Inception will suffice.

About installing apps out of store. Go to: Settings->Applications->Installation
and allow installs for non-store software.

Then just search the web for what you can find.

As a bottom line, N9 is just fine. If you feel it lacks in some area then search for some tweaks for that particular purpose apply them and see if you like it any better. Otherwise, don't worry about it and enjoy using your phone. Remember that hardware has it's limits and don't expect miracles. Harmattan is pretty impressive as it is.