Its fine with the compression you are running, look at it like this, your motor can only suck in some much air, give it the most it can take by increasing the TB size(to a point) you wont hinder your performance by increasing you TB size that much,

Look at the ITB setups, take the size of each one of those stacks and add them together, I bet is much larger than that 68mm you want to get. because thats what ITB's do, enable you to injest massive amount of air into each cylinder..

If the price is right get it, you may(may!!) see a small drop in TQ down low, but driving hard, how often do you see your revs go below 4K??

Now on turbo/supercharged setups size is almost null, due to the fact that you are working with positive pressure and at a point were your motor cant injest any more air than it already is, that boost is just waiting for its turn to get sucked in, so going from a 60mm-70mm will only see gains in spool up time more than it will with actual HP numbers..

hope this helps