Very complex question as every engine differs in how the perform but there are some general rules you can apply.
1) basically yes
2) No the boost pressure is going to stay the same so you are still only going to have the same amount of air available. To not loose power you need something else to happen like being able to run more ignition timing
3) Under boost you tend to put in comparitavely less LPG than you would petrol - explination to follow
4) In some turbo engine you will see more power on LPG than petrol keeping teh same amount of boost
We already know that LPG has a lower calorific value than petrol meaning for the same amount of "work" we need to use more fuel. Adding that extra fuel then allows less room for the air charge hence limiting the maximum power available on a typical engine so you normally expect to see a small drop in engine power.
This is still true with turbo engines but a few things happen in turbo engines that can reduce the power lost on LPG to even allowing you to gain.
Many turbo engines add extra petrol under boost for cylinder cooling so are overfueling. LPG does not offer this kind of cooling and adding extra LPG at those levels can cause engine running problems. We will lean the LPG mix back for correct burn thus resulting in the LPG taking up a little less space
The other factor is ignition timing. Some engines are set with very safe igntion timing and LPG will see no gain without the use of a timing advance processor. Other are fairly advanced and teh knock sensors retard the timing back on petrol and will then have more advanced ignition timing on LPG without the use of any extra parts.
If building a dedicated LPG engine then high CR and liquid injection is the way to go
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