Looking for topic to final thesis

That was my first thought when I first read the topic. Solid pick, to be sure.
Another topic you might consider is the effectiveness/consequences of different management styles. I wrote a graduate school paper about how managing least versus micro management. It was a fascinating paper for me, but I've been in both situations. I LOVE not being managed! My work is better.
Good luck!
"It is hard to fail, but it is worse never to have tried to succeed." - Theodore Roosevelt
"Public opinion is a weak tyrant to that of private thought." HD Thoreau Walden

 Mihkel,
I like the topic to measure how implementing the feedback affected the organization but it may be difficult for you to separate the effect you get from the feedback from the one-on-ones and coaching.  It may be better to evaluate the impact of the trinity (one-on-ones, feedback, coaching and delegation). 
How long is your thesis?  You will need to measure the impact which means that you will need to do a measurement before and after the implementation.  One way to measure is through an accessment of employee "satisfaction" with the work environment.  According to the book First Break all the Rules (http://www.themanager.org/HR/Book_Rules.htm), the best managers are those that build a work environment where the employees answer positively to these 12 questions bellow.  So you can do the questionary at the start (no trinity implemented) and at the end of the period after implementing the trinity.  I have no idea how you can measure the impact of the feedback alone.
Another topic which I would study if I had the chance is how implementing manager-tools recommendation can help drive the company towards sustentability.  This is just me because I am very interested in sustentability, which means operating at the intersection between what brings an economical, environmental and social positive impact.
I see it happening so often, managers which produce results but which results are not sustenainable in the long run because the interests of the individuals involved where not taken into the equation on the measurements taken towards the desired results.  So people leave companies, people burn out, people work without passion, etc, etc.  I really appreciate the way manager-tools recommends one brings into the work business more knowledge regarding the person´s personal life and interests, as I think that is key in employee retention and higher employee performance.
Please let us know how it goes and good luck with your thesis!
Nara
Questions to measure manager effectiveness:

1. Do I know what is expected of me at work?
2. Do I have the materials and equipment I need to do my work right?
3. At work, do I have the opportunity to do what I do best everyday?
4. In the last seven days, have I received recognition or praise for doing good work?
5. Does my supervisor or someone at work seem to care about me as a person?
6. Is there someone at work who encourages my development?
7. At work, do my opinions seem to count?
8. Does the mission/purpose of my company make me feel my job is important?
9. Are my co-workers committed to doing quality work?
10. Do I have a best friend at work?
11. In the last six months, has someone at work talked to me about my progress?
12. This last year, have I had the opportunity at work to learn and grow?

 Discuss how effective feedback is when tailored to the DISC profile of your directs.

 Mihkel,
You cannot implement the feedback without implementing one-on-ones for at least 3 months, then you need to go positive feedback for another three and finally negative feedback.  If i recall correctly  that is the absolute fastest manager-tools recomends you start on feedback.  You must develop a trust relationship with your direct Before start giving him or her feedback.  That trust relationship takes time and it takes one-on-ones.  Só i believe if you want to follow manager tools you need to át. Least implement feedback with one-on-ones.  So you will need time.  Well, you probably can go faster and do feedback without one on ones but i am not sure what the result will be and you cannot say you would be doing it manager-tools style because manager-tools does not recomend feedback without one-on-ones. 
Nara