Go visit some stores. 30'' above the cooking surface is just fine and doesn't obcure anyone's vision. Just make sure it's deeper and wider than the coking surface below, with additional CFM to compensate for the additional effluent drift that occurs with island venting that doesn't with perimeter venting.

I mounted mine 36" above my 4 burner range with griddle, and works fine. Out of head bumping and line of sight range, and I'm also 5'-11" ...

Since you are remodeling, you should consider putting your cooktop along the wall, not in the island. It is a frequent misunderstanding that the cooktop in the island allows you to interact with your family/guests better. Actually, cooking is only 10% of the time you spend in your kitchen. Prep is 70%. And anyway, while you are cooking you are looking down at your pans not at your family/guests. Also, when there is a cooktop in the island you have to make sure people sitting at the island are sitting far enough away not to get splattered.

Really, an island is so much better for prep. That way, you can have family members help you prep at the island if you want, and you can have people seated at the island without worrying about burns/splatter, and you don't have to figure out island ventilation. Keep in mind that not only is the ventilation going to obstruct your sight lines, but island hoods are more expensive to purchase and install. Also, your island hood will need to be more powerful than a wall hood, cause there are no walls or cabinets to help contain the smoke, so your hood will need extra cfms to make up for that.

The Kitchens Forum has a lot of people who enjoy helping with layout. If you wish to do away with your island cooktop layout, they can help you,

If you're looking for a hood that won't block line-of-sight, don't even bother with American brands (Vent-A-Hood, Viking, etc), they're too huge & boxy. Look at European designs - Miele 6290, Futuro Futuro Streamline, Elica Twin Optica, etc.

{{gwi:1410133}} Miele 6290 hood

{{gwi:1410135}}Elica Twin hood

Care should be taken to not confuse poser kitchen hoods with functional hoods. (Functional here refers to capture and containment of cooking effluent; poser refers to a kitchen environment designed for style.) The most functional hoods are those that emulate commercial hoods; the least functional have inadequate flow, or inadequate capture area, but often look very nice and have lower visual bulk.

The Elica Twin hood above, for example, appears to have a tiny aperture, and thus would not have high capture efficiency for high temperature cooking plumes expanding from pans on the cooktop below.

The OP must decide what is important and proceed accordingly, keeping in mind that performance, affordability, and aesthetics are usually conflicting goals.

kas

There are also units that are build into the counter or even the cook top itself. They look interesting but not sure how effective they really are. Some companies also have hoods that go into the ceiling. All of these options seem expensive, though.

http://www.bora.com/en/

http://www.gutmann-exklusiv.de/gb/home/home.html

Alex