I have owned all 3 types and wrecked out all 3 types or roof tops.The sunroof has the least room due to the strip/trim around roof lining.A hardtop will have a minor bit more room due to the roof lining being flexible.A targa top will give you all the room you need - if you want to stick your head out in the wind.
If your interested in putting a targa roof on your car,give me a yell.I have a targa roof cut out the backyard as a spare for mine.Will consider selling it.
Personally,I think trying to lower your seat would be the easiest/cheapest option.
I must agree with na/sdub...the seating position can be a problem.For me,I find that the rear of seat feels like it is a lot lower than the front of seat.
I will try to add a pic of an AW11 with lots of headroom.
Thank you....skip.