If you need it you need it is the answer. But I'm not sure you do.... That's really impossible to tell over the internet.
I have a 13yo TB with a very large high wither classic of most TB's and quite a flat back seen often in very young horses. Here's what I do for that horse. I'm not at all sure if this might work for you. First, I looked at his overall shape and asked people with horses as similar to him as I could find what they used and their horses overall satisfaction with that. While I was looking I invested in a few different very high end saddle pads. I have a grandeur, a few Equipedics, and a supracor. I like the equipedic the best and they come in shapes for english hunt seat, barrel, western, treeless, endurance and various sizes to fit your saddle. I use the endurance models as they are a bit long in the back and my small emergency pack can rest there. The high quality pad with the high density foam of the equipedic made a HUGE difference in his comfort. The supracor is a very thin pad and useful if your saddle you have does not accomodate more padding.
For him the biggest thing needed was high wither clearance. I looked at cutbacks and those with high wither relief. I wound up settling on a Thorowgood T4. This is a saddle maker out of the UK where tons of horses are TB's (as opposed to the more stock type of horse most popular in the US). I purchased this saddle because it was affordable (it's a synthetic - please do not poo poo it unless you have tried it). This was my first synthetic I purchased and I'm in love. Prior to that I was a Pessoa or Stubben fan. Not anymore. It seemed like it could work and the comments I found online were all very positive. This saddle is much more balanced than a wintec and the panels are smooth (not like the CAIR system). Thorowgood has a fitting system they sell you with the saddle called FISH. These are Fish shaped inserts you can plug in if your horse needs a wedge on one side for example, and it is possible to adjust the flocking inside though I have yet to need to. There is an adjustable gullet as well.
I would give this saddle a try before you go to custom. I say that because this saddle fits "TB's in general" (yes, each horse is individual but each breed has an average body type). See if you can find a used one. They would run about $400-500 used and $700-800 new. This brand really holds its value and so you could pretty easily sell a used one for what you paid if it did not work out. Also, investigate borrowing as many saddles as you can to test ride and test fit. Have a party at your house and ask all your horsey friends over for pizza and drinks and to bring their saddles. You start to get pretty good at sizing up the shape of the panels and such.