ghost, my 2c if i may:
1. your kick is asymetrical (relative to your body position)
your propulsion vector is not in line with your direction of travel (your body's centre line), meaning you kick behind your body instead of symmetrically below and above (ie down and up-kick). you can see that your down-kick is only going as low as your body's centerline, and your up-kick is well behind your body. in this way your kicks actually propel you at a slightly downwards directed angle rather than straight forward.
weight yourself neutral and adjust body position and you will find kicking to become more relaxed and symmetrical.
if you weight yourself neutrally buoyant (neckweight) you would have less of a problem with that since you don't need to kick yourself "down" to avoid floating to the surface.
also, your arms overhead position shows somewhat limited shoulder flexibility. not being able to keep the arms overhead AND stremalined in a relaxed manner leads to a compensation whereby the lower back is arched as you can see in the movie. this in turn makes kicking symetrically more difficult
your head position is slightly overextended. if you keep your arms at the side and drop the chin some more you wil have a less arched lower back leading to a more relaxed and symmetrical kick.
2. your kicks are shallow and fast
as has been mentioned. try slower and wider kicks without too much knee bend. you will have better fin control as well as less turbulence and drag. make sure your kick is symetrical and that you use the up-kick to counter the down-kick.
changing kicking style will certainly feel different as muscle effort is shifted to other groups and it will take a while to familiarize yourself with the new variation. don't take that initial impression as a sign of "inefficiency". remember, that your current style is something you are very accustomed to over many years of experience.
one thing that helps is to film yourself on ascent as this is usually the most naturally relaxed, balanced and symmetrical body position. compare your kick and body position on ascent with your descent and dynamic. ideally there shouldn't be any difference in form.
3. some thoughts on arms-overhead
drag increases exponentionally with speed. swimming with arms at the side will allow better upper body relaxation (o2 conservation). if you swim rather slowly anyways the o2 saving will be more beneficial than the extra little bit of streamlining as you don't disturb the water much anyways. if you sprint, on the other hand, you want to be as streamlined as possible, but it will cost you a lot of o2 to maintain a tight and streamlined body position.
seeing that you primarily spear you'll probably look for better dive times. extra relaxation in that case will probably be more beneficial for you.
4. what fin?
about your fin question, i'd always go for something softer rather than something harder. softer fins allow much better movement control with the benefit of great output from better technique. also, you'll be more relaxed in the upper body which gives you better o2 conservation and divetimes. hard fins are probably more for sprinters and real men
i'm primarily diving with bi-fins. even though i have c4 80's i usually use some old cressi gara ld's (the soft ones) as they are just fine. comfortable, reliable, care free, cheap and definitely good enough for easy, repetitive dives in the 20m range.
hope that made some sense
thoughts anyone?
roland