As long as the "custom" rifle is reasonably done by a competent gunsmith, having the new setup is never going to "hurt" you. However, having one may not always facilitate a noticeable improvement in your scores. It all depends on what the limiting factor is for your shooting prior to obtaining the new custom build. If the rifle was not the limiting factor, you may not see a noticeable increase in your scores right away.
One way to assess this is to shoot practice targets at 600/1000 under dead calm, or at least very mild conditions, if you can ever reliably get such conditions. If you can shoot cleans/very high scores with good X-counts (especially at 600 yd) using your current Remy 700 setup, it may be that your rifle setup is already performing adequately. But this is a very subjective method and the question you're really asking is one that may be difficult to answer definitively.
For myself, I use my initial load development at 100 yd as a rough guide. When I can reliably shoot quarter MOA groups at 100 yd with a developed load, I have yet to encounter a situation where that load and setup wasn't capable of winning matches at 600 yd or further, as long as I was on top of the wind conditions and/or didn't make any stupid mental errors such as crossfiring. If the best I can typically do with a particular load and setup is more like 0.35 to 0.4 or 0.5 MOA at 100 yd, then I'm probably leaving something on the table shooting it in a match. That doesn't mean it isn't possible to shoot a good score with such a setup, but your wind calls have to be that much better than everyone else's. Buying a custom setup is a big expense and unfortunately, it isn't always easy to know with certainty whether your current setup is the limiting factor in your game.
