Actually, I think you just consinder too far. 如此 以至于and 得 are similiar, I think. But as you say they have different usage in different context. eg He is so happy that he can not speak anything. In Chinese, 他高兴得说不出话 or 他太高兴了以至于他都说不出话来了。 eg He works so hard that he cannot sleep at night. In Chinese, 他工作如此卖力以至于晚上都睡不着。 他工作卖力得晚上睡不着觉（we usually use the former one than the latter, and in my opinion, I never talked like this because I think it is not right） 得 is used in a short one, and words after that can modify words before so. Think about 高兴得睡不着and 工作得睡不着（wrong） In most case, I think they can be replaced with each other, but I think 如此以至于has a wilder range in usage. 如此以至于is like a sentence, and 得is like a phrase. However, I do not know if it is right, just depending on my intuition. I think you can read more and talk a lot, because I really do not the real difference. I know how to use in that people say like this.