Hi! Actually, output goes up by 3dB for every doubling of power.
https://jlaudio.zendesk.com/hc/en-us/articles/217201737-Doubling-Power-vs-Doubling-Output
Sensitivity is one factor that you can use to evaluate speaker choices, but it's not the only factor. Some aspects of speaker design that improve sensitivity may sound worse. One difference between a dust cap and a phase plug is that the dust cap adds output (increases the sensitivity rating) but usually does so at the sacrifice of frequency response in the upper octaves.
In general, your thinking is correct that having a speaker system with higher sensitivity will be louder on a head unit. You can only plan on 5W being usable out of a head unit, all channels driven. Beyond that distortion becomes audible, and quickly. You should still be able to achieve a 93-95dBA output level which is plenty for HU power.
So, assume you find a speaker system with 90dB 1W & 1M and you give it 4 clean watts of power - you can expect 96dB 4W @ 1M. In a car, you sit about 1 meter away from a speaker and about 2 meters from the rest. Overall output would be the logarithmic sum of all four speakers, getting you close to 100dBA of possible output - theoretically assuming no cancellation occurring.
My recommendation for piecing a system together is to buy what you want at the end, in steps. Sacrificing now and getting something cheaper isn't saving money if you plan on buying the better version later. Saving up for the good item is worth the wait (or finding it used for a fraction of the cost).