Yes, that's the deal with magnums; the extra 100-200 fps costs powder. Look at the 7mm STW - load data shows about 10 fps per grain of 4831 above 7mm Rem Mag loads. Do you need it? You have to decide for yourself.
I'm assuming by fawn you mean little tiny one with spots born later in the year. No. Did it before and would try not to do it again. I didn't see the fawn until after the doe was shot; it bleated all night and into the next morning. I believe it took up with another doe with a fawn on the farm...
Millsworks,
This threads a bit old but I wanted to give you my experience with a similar project. I had a Weatherby Vanguard in 270 Win that was a bit superfluous with my other rifles so I rechambered it to 270 Wby. It has a 1:10 / 24" barrel and it was well worth the expense. I bought a Magnum...