Whiteflash is a super ideal cut company. The prices will be more expensive by 20 to 40% compared to generic GIA diamond as super ideal cut requires carat weight to be sacrificed for a better shine. 

If you are aiming for top quality diamond then it makes sense to look at super ideal cut. But if you are not very particular and just want a diamond ring, then you should just go for GIA diamond. 
Actually there is always this misconception that buying diamond overseas will be cheaper. In fact, the local jewellers (those with GIA listing online) are now very competitive that even the biggest US online player, Blue Nile is not that cheap. 

For example a GIA 0.3 carat H VS2 3EX None with starting price of 18k setting:-

Baemont                      : RM2,069 + RM1,399 (setting starting price) = RM3,468
Diamond & Platinum      : RM2,170 + RM900 (setting starting price) = RM3,070
Lavin New York              : RM4,050 (inclusive of simple setting) 
Zcova                          : RM2,180 + RM1,650 (setting starting price) = RM3,830
Michael Trio (Singapore) : RM2,274 + RM1,584 (setting starting price) = RM3,858
Blue Nile                      : USD 421+ USD 600 (setting starting price) = RM4,850

*Very important to look at setting price too as some will charge cheap for diamond but high for setting (ie Blue Nile). 

Of course, this is a simple search based on 4Cs, without taking into account the type or location of inclusion, the actual proportion etc. But you get the idea. All of them is cheaper than Blue Nile.

My apologies i made a mistake in my previous post. The RM4,050 (inclusive of simple setting) is Tailored Jewel. Lavin New York is RM2,243 + RM970 (setting starting price) = RM3,213.

This is the correct one:-

Baemont                        : RM2,069 + RM1,399 (setting starting price) = RM3,468
Diamond & Platinum      : RM2,170 + RM900 (setting starting price) = RM3,070
Lavin New York              : RM2,243 + RM970 (setting starting price) = RM3,213
Tailored Jewel                :RM4,050 (inclusive of simple setting)
Zcova                          : RM2,180 + RM1,650 (setting starting price) = RM3,830
Michael Trio (Singapore) : RM2,274 + RM1,584 (setting starting price) = RM3,858
Blue Nile                      : USD 421+ USD 600 (setting starting price) = RM4,850

*But just to note these are not all the engagement ring companies in Malaysia. I'm merely stating those that have the generic GIA listing online. There are still the bigger retailers like Tomei, Habib, Lazo Diamond, Degem, SK, Love and Co, Tiffany etc, which offers various type of diamonds (could be ideal cut, super ideal cut or even lab grown). Some also offer very good deals for GIA but may not list them online. 

Anyway, unless the location is too far from you, if not it is indeed important to be able to view the real diamond with your own eyes as sometimes, certificate does not tell the full story.

And yes i think Lavin New York is a good place to visit as I have heard good reviews about them. But my recommendation is to always visit a few more different jewellers to see which one can give you a better offer (including recommendation based on diamond proportion, after sales service etc). You never know what you are missing out from others if you only visit one store.