First off thank you paperslayer, you obviously have played with long barrels and strong cases!
We all know there are fast barrels and slow barrels out there, but working in the normal quality 29" barrel and especially using the 284 case 3000 fps with a 168 Berger should be easily attainable. I have played with several wildcats on the 284 case and currently own 3, if you have never played with the 284 family of cartridges you cannot know what a strong, efficient little case it is. It will SAFELY do things the loading manuals will never print, because of the 88 and 100 wins out there that it is most commonly found in.
In a good bolt action long enough to seat bullets out properly, with a good barrel, it will outdo the 280 Rem by nearly 100 fps SAFELY.
My estimation was not based on loading manuals or internet forums or interpolation, but on real world experience of more than 25 years of playing with this particular family based on the very unique 284 Winchester case. Also the powders suitable for this combination are in the burn rate that will make use of that extra barrel length significantly, not marginally.
I have owned and played with and loaded for over the years.......22-284 (current) 4 different 6mm-284 (current) 25-284, 6.5-284, 284, and breifly with a 35-284......in barrel lengths from 21"-31"
Douglas