Speculation or facts? Please provide hard evidence.
I have not found any hard evidence so far. Most testers have only shown the standard ATI poster showing 8 pipes instead of 12 or 16. This says nothing. Other testers said that the MR9800 has only 110,000 transistors instead of 160,000 transistors (desktop x800). But again: this does not say too much since the difference may result from not taking into account the transistors relating to the deactivated pipelines (if any).
On the other hand, it may well be possible that the MR9800 has 16 pipes since (1) it is based on the x800 architecture and it would be much more expensive/time consuming to create a competely new chip than to simply deactivate a few pipes (which also helps to reduce heat and power consumption), (2) the MR9800 was released fairly quick after the x800 came out, and (3) there was always the rumour that there will be also a 12 or 16 pipe version of the mobility x800/MR9800 which would also support the assumption that the core of the 9800 and the x800 is almost identical.
If the MR9800 really has 16 pipes I do not expect them to be defective. Mobile parts are generally more carefully selected than desktop parts (i.e., they only take the high quality parts). Moreover, the x800 design is not that new anymore. Probably, there are not many defect chips produced anymore.
However, if the die size of the MR9800 was really smaller than that of the x800 ... well this would be some kind of evidence that there are indeed only 8 pipes. Of course, an official statement by ATI would also be fine.
Anyway, I agree that trying this BIOS trick is dangerous. Maybe the next version of the bios editor RaBIT will support the (de)activation of pipelines.