Well, let's face it.
If you want to use a Japanese premium ship, this is what you have:
1. Yubari, T4CL - Great for low tier AA, but not exactly a good premium ship you can use to get good credits and train captains of other ships due to the chaos nature of low tier battle which will most likely sour your experience. Not to mention with 2 not so fast turning guns in a gun infested environment (US Cruisers, Kuma, etc.).
2. Iwaki Alpha, T4CL - Truly awesome, but Alpha tester only.
3. Ishizuchi, T4BB - We all know what BBs have to face in low tier battles.
4. Atago, T8CA - If you can play average on a cruiser, it will do much more than above ships, making it the best premium ship you can acquire in IJN at the moment.
The only problem you're facing now is you're most likely to encounter the fastest grinding players as a T8 ship.