I'm currently thinking about starting to learn Mandarin and how to best use my existing Kanji knowledge. I suppose that you can save a great amount of time but just not "relearning" characters with a same/similar meaning, especially since I'm learning readings and nuances through vocabulary anyway.
Which leads to the question whether or not there is a most common xk Hanzi list/deck in which characters that are used with the same meaning in Japanese (and contained within something like the JIS第1+第2水準) are filtered out?

well as someone who used rtk to learn japanese, and is now studying mandarin having recently finished rth, i'll give my opinion... (i'm learning simplified btw, might study 简体字->繁体字 at a later point)

I think that most characters are different enough that you ought to relearn them altogether as a group. also the meanings are usually different. which is not to say the original rtk meaning doesn't apply, but that it often would become a 2nd or 3rd most common meaning, and there's another that would be more helpful to learn.

having done rtk, straight rth was a breeze... totally new kanji only account for about 1/4. so i found a shared anki deck for rth that included character pronounciations (chinese only has 1 pronounciation for most hanzi) which made redoing the whole process much more useful. that would be my recommendation on how to proceed.

it was hard but super rewarding. now 1 year later i've finished that deck and some others, as well as the NCPR textbooks, and with a little help from a dictionary I can read simple articles with no problem.

Did you already link that deck somewhere?

Also, what did you use as a text. I am starting from zero, playing with iknow.jp (on a free 3 month signup), and simply have no idea where to look for advice.

stuff like IMEs, basic grammar points, etc. I actually don't even care about speaking

I'm sorry I forgot which deck it was, but there was one that some guy made which matched RTH to character pronunciations for the first book only. And then I found a second deck for the second book and added it to the first.

learn with pinyin as the phonetic system, and use a pinyin keyboard. It's not english but is easy to read... They have a few kana-like phonetic systems, such as bopomofo, but pinyin is used pretty universally.

After finishing rth1 with pronounciations, I used the new chinese practical reader books (there are four I think...) they were pretty good but you may find others you like more. After that I went back and finished RTH 2 and a few small decks.

To practice simple reading there is this very good website called the chairman's Bao, which is like NHK easy, with simplified short articles.
http://www.thechairmansbao.com

And chinese skitter is an awesome way to drill vocabulary. It's a little expensive but with a 1-year subscription isn't unreasonable given the tremendous value... there's nothing else like it.

Also the dictionary Pleco for iPhone is a pretty amazing all in one. Several E-C dictionaries and a C-C dictionary. An OCR, flash card program, speech-to-text voices and more.

And if you benefit from koohii, you should probably frequent chinese-forums.com for similar discussions.

good luck and enjoy chinese!