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I'm not sure it's possible to downgrade from 2.5 to 1.5.
The downside is that first, you might fuck your firmware (essentially fucking the PSP). Second, you miss out on all the added features from all the subsequent firmware updates. Such as web browsing, Flash compatibility, better file support, RSS feeds, etc. Third, you won't be able to use many of the planned or rumoured accessories (such as webcam, VoIP, etc). Fourth, you won't be able to play games that require newer firmware.
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