flac question

Yes, the higher compression rates take more power to decode, I've never had a problem with level 8 compression on the portable players I've used though. Currently a rockboxed iMod.
I see lower compression rates as a waste of space, so the best bet is probably to test and make sure your player has no problem with level 8.

-Yes, it only affects the time to process the file.

-I think the higher compression setting only takes more power to encode but is almost the same to lower settings when decoding.

-Personally, I always compress at level 8.

The different flags (0-8) have these affects:
* Compression level 0 = larger files, 8 = smaller files.
* Compression level 8 = faster encoding, 8 = slower encoding.

They have minor affect on decoding time though. Which have been the goal of the developer.

With the original FLAC Encoder there should be no Problems with -8.
Even with Flake (alternative FLAC Encoder) you can go without Problems up to -10, but -11, -12 and -99 are too much for some portable devices.

So Flake goes to -99? How small do file sizes get with 99? How does it compare to monkey's insane?

Well it depends on the music of course.
Flake is the FLAC encoder that is also used in FFmpeg and it goes to -12, but there is also a hidden experimental mode that is -99 which gives the best compression.
I did a little comparision, here are the results that I got from a short track that a friend did compose:
Original Wav File : 16.397.324 bytes (100%)
Shorten : 10.815.229 bytes (65.96%)
Apple Lossless : 10.294.163 bytes (62.78%)
FLAC -8 : 10.165.271 bytes (61.99%)
Flake -99 : 10.036.912 bytes (61.21%)
TTA : 10.017.700 bytes (61.09%)
WavPack -hh -x6 : 9.850.046 bytes (60.07%)
Monkey Audio extreme : 9.763.716 bytes (59.54%)
OptimFrog --experimental : 9.757.201 bytes (59.50%)
OptimFrog --maximumcompression : 9.667.377 bytes (58.96%)

Encoding Speed: Shorten > TTA > FLAC -8 >> WavPack -hh -x6 > Flake -99 > OptimFrog --experimental >>> OptimFrog --maximumcompression

ALAC and Monkey Audio have been packed with Max, so it's not fair to compare the speed to the CLI encoders (which are faster). Anyway: ALAC was a little bit faster than FLAC -8 but definitely slower than TTA, Monkey Audio was a little slower than OptimFrog --experimental.

OptimFrog --maximumcompression gives the best compression ratio, but is unusable (unless you don't mind that a 1:32 min track takes about 10 minutes to encode on a iMac G5 2.0 GHz).
IMHO Flake -99 isn't worth the extra encoding time when you can get better compression results with TTA or WavPack in less time.

Note: This was only a short test. For a serious test you have to use different genre tracks and repeat the time taking several times.