Such good use has been made of his spare time (as) his Engli

我觉得就应该用that
分析：
原名变为主动：
He has made such good use of his spare time that his English has improved a lot.
他利用业余时间是如此充分结果他的英语进步很大.
这里的such...that...(that引导的是结果状语从句,所以用that,不能用as.