Re: .338 lapua improved or 338-378 wby improved
I have 2 338 Khan's which is a 338/378 improved. One is built on a WBY MK IV action with a 27" Lilja, the other is built on a Nesika Bay action with a 30" Lilja.
You will have no trouble getting 3000 fps with the 300 SMK and a 30" barrel. I have been shooting 250 SMKs out of the Nesika at 3350. This hunting season I have been using the WBY action Kahn loaded with 300 grain Bergers. It easily pushes the Bergers to 2875 which is where I have been loading them to due to the "nose slump" problem when pushed too fast. I was getting around 3130 to 3150 out of them before that issue came up and I throttled them back. Both my Khan's are very accurate (1/4 moa) and both were built by Kirby Allen (Fiftydriver). My rifles are quite heavy compared to what you are wanting to build, both coming it at over 16 lbs (with scope). As light as you are trying to go, be sure to use a brake or recoil will be a bear.
Now, as far as the Lapua improved....... I have no experience with it although from what I have seen, the performance should be about the same as the 338/378 Imp. Brass will be expensive for both so no advantage one way or the other there. However, the Lapua brass is a harder alloy and may give you more loadings if you are going to be pushing things to the limit. I can get up to 10 loadings from my brass if I take it easy on it (like my current 2875 load with the 300 Bergers). If I start pushing the limits the primer pockets will get too lose to use after about 3 loadings. Normal, middle of the road loads will give me about 5 or 6 loads. This is true for either WBY or Norma brass, both of which are known to be quite soft.