In any case since no one has ever taught me how to do a proper atemi, I decided that I need to train that elsewhere. For this reason I am considering taking additional boxing classes or another striking (hard) martial art.
I know there are always mixed views on this but I am mainly interested in hearing what the aikido purists have to say about that. Especially what they would recommend, if there is something, to do inside the framework of Aikido.
All the best.
Like you say, to get good at hitting you need to hit. "Atemi" can be more than hitting hard and well, but that's the aspect it sounds like you're focusing on. Not everyone puts much emphasis on hitting and so where it's not as emphasized as you'd like, you'll have to fill in to the degree you like. Your post reminded me a bit of this song: http://www.youtube.com/watch?v=xbFU6w7q3dQ
So my advice is that when you're on the mat you do your best to understand and respect the effort of your teachers; beyond that, do as you mentioned and supplement; keep trying to find people who are willing to train off the mat with you more in the way you're looking for. The style of martial art ultimately means very little; we're all individuals with our own abilities and understanding, regardless of whether or not we share the same nomenclature or even lineage. Some schools will have very strong hitters because they work on that; some won't. As students it is up to us to find our way...and it sounds like you're doing that as well as most since you're identifying potential problems and addressing them. Maybe your opinion will change over time like mine did (although to be fair I have never been interested in hitting hard, so our goals are quite different).