We have been creating tons of profiles for the L25500 for more than a year now. We are working as a Global R&D partner with HP on this particular technology.
Profiling this printer is not easy, in fact it is probably the most the difficult machine we ever had in our profiling lab (we have around 30 here). Once it runs and you know what to do, it does a great job and the quality is high.
Couple of things you need to pay attention to:
1. Use a low amount of light inks. When using ONYX X10 set the gamut size in ink restrictions to coated (this drops the light inks to 20%) depending on your media you might even go lower.
2. Make sure you make some big prints with 'difficult' color bars to see if you temperature settings aren't causing vertical and horizontal banding. We use green, blue, red and a composite gray bar on the full media width and around 20" in height to judge this.
3. The curing temperature is the one that reaches the highest temperature. In case you see issues with the media (waving effect, wrinkling etc.) drop this temperature.
4. In case you see dark banding or coalesence increase the heat airflow setting. The settings HP recommends in the RIP driver are for categories of media, you might need to adjust them.
In general: try to get the light inks down and try to avoid to high temperatures.
Hope this helps a bit. I strongly recommend you to use X10, this makes profiling this printer a lot easier, compared to 7.x. In case you use 7.x you have to enable the OEM Dot pattern and configure your own light ink curves. This is not easy and a trial-and-error process. I attached 1 curve you can import in the OEM Dot pattern dialog, this is a curve we created here and it gives pretty good results with most media. Make sure you set the ink restriction for your dark inks also from this dialog!! Before printing the test file make sure you CMYK (dark inks) are all set to 100!
In regards to the ICC profile:
you can use a pretty low blackstart (between 15 and 20) and a GCR High curve. Since the black drops are pretty small the effect of a low black start is hardly visible. It will give a more stable gray balance over a longer period of time.
In most cases you will need to use the Advanced Inklimit. Read the ONYX documentation on how to use the BIC (Black Ink Compensation) since you will need it. Especially on (non photo) paper based medias (like blue-back and uncoated paper) you will need it.
Got to learn a new RIP on top of it. We'll work it out.