Yes objective makes a difference but the type/quality of lens used and coatings make up the meat on light transmission. The better the glass and coatings the less the objective will make for usable light transmission. Scope with superior coatings with smaller objectives will out perform those with larger objectives and low quality coatings. Comparing apples to apples it should be bigger objective more light but the human eye can only take some much light and the rest is wasted so it will become a point of diminshing returns. For most application good quality scopes of 40-50mm is all a biggame hunter needs for legal hours. By putting a large objective scope on you may change your shooting position which can result in poorer accuracy and scope crowding(which can lead to the ouch of be SF'd). This is subjective as some find med mounts to be perfect, while others find lower or higher to be aok. I just say this point so you take it into consideration before you purchase any scope(mount height and what it may or may not do to your shooting position.)

It is argued that a 30 mm should return more light than a 1", however the real benefits are internal strength and adjustment gains in the 30mm vs 1". The real difference is as stated above to get light transmission gains, not tube size. The advantage in tube size is so slight I would venture to say it is almost not measurable in light transmission on comparable glass and coatings.

The diavari is superior in glass and coatings so it should be brighter, clear, etc. I also would venture to say it's component make up would be more superior. This would account for the price difference. Is it worth it? Only you can make that call. Personally I wouldn't buy the diavari as the Conquest is all the scope I need for my hunting needs. I also would never buy a 56mm as my 40-50mm objectives of different manufacturer's is all I have ever needed for hunting in low light(legal times). While some are brighter not even the smaller objective's on my cheaper scopes have lost me in the field. Yes, some days it is too dark to distinguish and is legal but part of hunting is waiting for confirmation and that can mean not taking a shot, it is never a mistake in my mind not taking a shot that your not 100% confident in all aspects. I doubt in those rare cases any scope would have bought me the confirmation I needed to make the shot...I may be wrong just my opinion. Again it is personal and depends on your situation.

I would say do some research and take some boo's through the scopes available for your needs. The man who holds the check hold the desicion and everybody is different in what they need, want or see benefit in. Good Luck.