Waring's problem (previously asked about here) asks, for each integer $k \ge 2$, what is the smallest integer $g(k)$ such that any positive integer can be written as a sum of $g(k)$ kth powers. We have $g(2) = 4$, $g(3) = 9$, etc. A (harder) variant asks what the smallest integer $G(k)$ is such that all sufficiently large integers can be written as a sum of $G(k)$ kth powers.

I have two related questions:

1. What is known if we relax the condition ``any positive integer'' and only require a positive-density subset? More precisely, we look for the smallest $g'(k)$ for which there is some $S \subset \mathbb{Z}_{>0}$ of positive density such that any $x \in S$ can be written as $g'(k)$ $k$th powers. Then we have $g'(2) = 2$, while $G(2) = 4$; and $g'(3) = 4$, while it is only known that $4 \le G(3) \le 7$. Is anything known about $g'(k)$ for k = 4,5, or larger?

2. For fixed k, is there an efficient algorithm that, given n, writes n as a sum of $g(k)$ kth powers? What about decomposing n into the minimal number of kth powers for that n? (Here `efficient' means polynomial in log(n).)