Hi and welcome to the forum. I had an IS220 and was planning on trading it in for an IS300h but took a GS300h for a test drive and was smitten and now have one. In general I feel that the GS is a down market LS rather than an upmarket IS. The difference in quality is noticeable. I have the premier of 2014 and can't think of anything I'd replace it with.
More specifically, in regards to your questions here are my thoughts.
1. It's a different beast entirely regarding quality and feel good factor. Not as 'sporty' as the IS but in real world terms not a slouch either. Definitely a more relaxing and comfortable proposition.
2. I've had extended warranty since I bought it and have never needed it but it's a great peace of mind. Like any insurance you hope you never need to use it. But if something did go wrong it would be well worth it. I have 28 way adjustable front seats. If they were to pack up I'd hate to think of the cost!
3. No real known issues except maybe the shocks but this is covered by extended warranty.
4. Not cheap but similar to IS.
5. Go for the premier ! It's lovely and luxurious.
Good luck in your search!