>> Any drawback for using 8.0 instead of 8.1 (e.g. performance)?
There are no performance issues, but there are advantages and disadvantages between 8.0 Silverlight based apps versus 8.1 XAML based apps.
Regarding 8.0 Silverlight based apps:
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Advantage: They’re supported on both Windows Phone 8.0 and 8.1.
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Advantage: Has more 3rd party library/sdk support, like AdMob.
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Advantage: Microsoft incentive programs usually require this. (Although this will likely change in the future.)
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Disadvantage: Does not support universal apps. (ie: No Windows 8 Metro or Windows RT tablet support.)
Regarding 8.1 XAML based apps:
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Advantage: Has universal app support. (ie: Windows Phone 8.1, Windows 8 Metro, and Windows RT tablets)
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Disadvantage: No Windows Phone 8.0 support. Only 8.1. (8.0 takes nearly half the WP8 market right now.)
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Disadvantage: Not a lot 3rd party library/sdk support for it yet on Windows Phone, such as AdMob.
Now, the biggest reason you want to use an 8.0 built app is because it makes your app available to most of the Windows Phone market (ie: 8.0 and 8.1 devices). According to the last statistics I’ve seen (see link below, page 6), you’ll see that 8.0 devices take about 45% of the market. If your app was built for 8.1, then you would lose 45% of potential customers. That’s huge!
http://blog.adduplex.com/2014/09/adduplex-windows-phone-statistics.html
So, Microsoft and the WP8 native development community is currently going through a painful transitional period. At the moment, most native WP8 developers are sticking to 8.0 Silverlight and will later work on 8.1 XAML support in the future. This is Corona Labs’ plan too. Just note that native developers don’t “add support” for 8.1 XAML. Native developers (such as Corona Labs) have to rewrite their entire UI code base to support the new universal XAML framework. It’s a large undertaking… which is also why you don’t see Google currently supporting 8.1 XAML yet for AdMob, because it basically means that they have to rewrite it.
Anyways, I hope the above helps explain it.