It should work well, because our engine doesn’t use any plugins or supplementations than the browser itself, and PhoneGap just uses a WebView to show the web page. If your HTML5 version works well on the device you want, it should work with PhoneGap.
But the WebView doesn’t equals to the native browser, for example on iOS, the UIWebView is not the safari, it may have problems like non retina support. And the performance is equal or worse on WebView than on the real browser.
You can also try out JSB binding which will bind your cocos2d-html5 code to the native cocos2d-x code. In this way the performance of you app will be greatly improved as a real native application.