If you don't learn Pinyin or Zhuyin or any other system for phonetic transcription for Chinese, how would you be able to look up words in the dictionary? It will be easier for you to learn Hanzi, but the pronunciation is completely different to Japanese. And the characters are not always the same and there are many false friends.

Let me give you some examples:

Japanese:発, 学, 説

Simplified Chinese:发, 学， 说

Traditional Chinese: 發， 學, 說, 

So, sometimes there are three different versions of the same character. Sometimes the Kanji resemble the simplified characters and sometimes the traditional ones.

False friend:

Japanese: 手紙 letter

Simplified Chinese: 手纸 toilet paper

There are also Chinese characters, which don't exist in Japanese and vice versa.

Japanese: 畑 field (doesn't exist in Chinese)

So it is not like that, that you can just learn some basic grammar and then fill in the characters you already know.

They often use the words differently:

tree, book, dog, to write

Japanese: 木、本、犬、書く

Simplified Chinese: 树，书，狗，写

Traditional Chinese: 樹, 書, 狗, 寫

When I started with Chinese, I could also already read Japanese, so indeed that helped for learning Hanzi and reading in general quicker. Knowing Japanese helps learning numbers in Chinese and understanding the system of Chinese classifiers. 

Here is another thread about learning Chinese advice: https://www.italki.com/question/343637

Some recommendations for apps and weblinks:

"Hacking Chinese" is a very resourceful website with plenty of tips about "how" to learn Chinese:
http://www.hackingchinese.com/
"Chinese Forums" is an active Chinese language learning community: http://www.chinese-forums.com/
"Pleco" is THE dictionary for your smart phone and tablet: https://www.pleco.com/ ;

All Chinese children learn Pinyin at school and the schoolbooks for first graders are annotated with Pinyin:
http://www.pep.com.cn/xiaoyu/jiaoshi/tbjx/kbjiaocai/tb1s/201009/t20100915_895233.htm

Here is an example of a dictionary. I looked up the word 头发. The dictionary shows the reading in Pinyin (tóu fà) and Zhuyin (ㄊㄡˊ ㄈㄚˋ), but Zhuyin is only common in Taiwan.

http://www.zdic.net/c/4/151/335837.htm

If there is a discussion about how to read a not commonly known character, Pinyin is also used:

芈怎么读？　读[ miē]，[ mǐ]　其解释有两层意思。
http://bbs.cssn.cn/forum.php?mod=viewthread&tid=202690

And if it is simply not possible to type the given character on a computer because it is too uncommon or too new, like the new character “duang”, then Pinyin is also used:
http://www.chinanews.com/cul/2015/03-01/7089436.shtml
(even if someone can’t read Chinese, he can easily see, that this article is about “duang”)

https://en.wikipedia.org/wiki/Duang

So long story short: Learning Pinyin is definitely no waste of time. ;-)

Do know how to pronounce the sounds of Mandarin when given the character?

Can you differentiate tones on listening and speaking?

To be honest, pinyin is a helpful even if you speak another language. But, you can try a few lessons without pinyin to see how you cope.

Knowing characters helps you learn faster overall but can you extrapolate that rate into learning without pinyin and maintaining that rate of progress? Honestly don't know. I use it pinyin as an aid but not a crutch. I even speak another Chinese dialect and still learnt pinyin for Mandarin - it isn't very hard at all.

My advice is to go through the basic progress like any other person and it's probable you are faster than most other people. Don't assume because you can read faster, you will automatically have the same rate of improvement in speaking or listening.

Hi Jason here's some of my ideas. Hope that helps. First, Japanese is a different langauge from Chinese.

Even though I can read a lot of characters in Japanese which look similar to Chinese ones, that doesn't mean that I can understand Japanese. Some of them mean very different things.

For example, 手纸 in Japnaese means letter whereas in Chinese ite means toilet paper.

So, I think your Japnese level wouldn't help much in terms of that.

By the way, I think speaking and listening is the basic part of communication. I strongly recommend you to start with pin yin. I would like to add you as my friend on italki. Thank you!