Honestly, I'm coming around to not ever doing custom shirts again, or at least not expecting much from it. Unless maybe I could do them on Saville Row. I got started with them in the first place for a handful of reasons. One, the sleeves are often too long for me when I buy off the rack. Two, I bought into the idea that 'custom' absolutely meant higher quality. Three, I liked the idea of having the control of picking and choosing all the options. Four, once I moved to Asia from the States I was intoxicated by how affordable they were.
I now would reverse course on much of that. I've found a good alterations shop that can shorten the sleeves on shirts I buy off the rack. I know that the quality from any and every shop I've dealt with thus far in Asia is far, far below the quality from one off-the-rack shirt from a solid Italian or Japanese maker. I now regret the 'custom' choices I made on some of my shirts, such as contrasting fabric inside the collar and placket. And I have thrown away more 40-dollar shirts than I want to admit, while I keep and enjoy wearing my 100-200 dollar ones I have purchased carefully.
Put simply, the Bangkok guys can make a very nice department-store-quality shirt, if that is what you are after. You will pay around 30-40 dollars for it, typically, which is also what you would pay at a department store. You can choose your options. The fabric may not hold up as long, though, and you may not get the same level of workmanship.
What I'm coming around to is this. If you really want quality workmanship and a shirt you will be proud of wearing, buy something nice and have it altered if need be. In KL your best option, as far as I know, is the P Johnson line from Australia at WJ&Co. Super solid shirts at a price that won't break the bank. Or go on Ebay for Italian makes that you can nab all day long for about a hundred bucks. Put either of those shirts on your back, and you won't be terribly excited about going 'custom' in KL again.