kernel upgrade vs dist-upgrade

[ilisepe1]

Hello,

Let's say that we have a system with debian 5, that we would like to upgrade it, in order to run it on new hardware.
What would be the most preferable solution? 3.6.x kernel upgrade (vanilla compile from kernel.org) or dist-upgrade?
We tried the former solution (because it would be much faster) and worked fine. The system was performing ok, until we did a pgbench where the results were disappointing (much slower than the previous kernel, almost half!).

By googling it, we saw a lot of posts saying that due to a scheduler/spinlock change (with kernels >=3.5.3), pgsql would have a 20% performance hit. We tried other kernels too (3.2, 3.4, 3.5.2) but they were doing exactly the same problem.
So, it looks like this "bug" was not the problem in our case.
The whole situation rose a thought whether upgrading just the kernel may not be enough after all and that the best thing to do is a userland upgrade as well.

What do you think about that?

Thanks