the best you can do is to buy an A8 Pro from a user, much cheaper than buying from the store. A8 is much faster, especially if you use terrain lod with pro. And you can make nice dynamic shadows too... wmb has an entity limit of approx. 3000, so you can place grass and vegetation using an own load/save system, e.g. in MapBuilder I could achieve nice enough quality and performance. setting entity lod and clipping properly is a must.

moreover this game reminds me to Solus, a recent game made with UE4 (it looks really great): http://www.hourences.com/thesolusproject/