Now- let's sort out your eligibility to the HKPIC (permanent ID card) first:
1) If you were born in Hongkong as a CHINESE CITIZEN, then you qualify to be HK PR. Your taking of the "US Citizenship" does not affect your PR status or Chinese citizenship while in China (including HK).
2) If you were born as a NON-CHINESE, presumably when you were born,you would be considered a HONGKONG BELONGER (just like Chinese people born in Hongkong), or that you would have had the Right of Abode (ROA) in HK.
According to the Immigration Ordinance, a non-Chinese PR would lose the ROA if he/she is absent from HK for over 36 months (or NOT ORDINARILY RESIDENT in HK for over 36 months)
Nonetheless, you can still have the Right To Land, which is the same as ROA, except that you may be deported under certain circumstances (e.g. committed serious offences)
--> THUS, you may qualify for ROA (PR) or RTL (essentially same as PR)
Answering your questions:
1) PR status or RTL- allows you to work, study, live in HK without gaining any visa/permit
In addition, being a PR- may entitle you to
* certain welfare benefits
* Right to vote in District council elections, and the Legislative Council election (and possibly elections for the Chief Executive)
You also have the right to be elected as a member of the District Council, and limited right to be elected as Legislative COuncillor
* As a PR- you cannot be deported by the HK Government under any circumstances
* I heard that as a HK PR (and if you're non-Chinese)- you might receive some beneficial treatment/discounts when obtaining a mainland (Chinese) visa [not sure about this, you should check with China Travel Services or the Foreign Ministry Office in HK]
* If you are HKPR and a Chinese citizen- you're also entitled to the HKSAR passport, and the Travel Pass to the mainland for HOngkong and Macao Residents [which is issued only to Chiense citizens living in HK and MACAO]
2) Drawbacks-? Not sure. Shouldn't have any drawbacks, apart from that you would have to fulfil the duties of a PR in HK (imposed by law)-
I don't think in respect of residents' duties, the burden on HK PR is heavier than on non-PRs.
[However, note that in 2003, the Govt introduced a National Security Bill, whch, if passed, would make HK PR liable for acts of secession, sedition, etc. conducted outside HK, regardless of nationality. However, the Bill was not passed]
3) Nothing happens to your US citizenship by applying for PR in HK. PR is only a residence status, which has nothing to do with citizenship in itself.
4) Yes, as a US citizen (and presuming you're non Chinese)- you need to make your apply for the HKPIC in Hongkong.