This is a suggestion. I have 7 sites now using X-Theme. I have to reduce the massive default header sizes every time with custom CSS. Would it not make sense to replace the letter spacing slider with a field that can set the font size for each header? Font size and letter spacing side by side would be very useful, where as the slider itself is not necessary in my book.
We certainly appreciate the feedback! This is something we can add to our list of feature requests. This way it can be taken into consideration for future development. All of these items are discussed with our team internally and prioritized based on the amount of interest a particular feature might receive.