The only thing is, for a spring pack to work properly (i.e. be able to actually use all of its travel) the shackle length has to be correct as well. They can build you a set of springs that has so much lift, but if you want it to be flexy as possible, then you'll need the right length shackles. They should be able to tell you the minimum shackle length you need to get the full flex out of the spring packs. Anything over this is considered pure lift, and not any more or less functional for the spring packs.
If they can build you a set that has the lift you want and that works with your stock shackle, then that would be the route to go.
If you still want to retain the stock shackle, even if they limit the travel, that shouldn't be a problem for everyday driving. If you plan on taking it offroad and want all the flex you can get you have to engineer the whole system as one.