Trident 4 issues with jquery (performance)
Trident with IE8 have some performance issues with jquery. Is there any way to upgrade the rendering engine from v4 to 5 without upgrading to IE9?
No, there isn't any way to upgrade IE8 to use IE9's rendering engine without upgrading to IE9.
What you're asking for would effectively be an upgrade to IE9.
It is, however, possible to use the IE8 engine (or the IE7 engine) in IE9, so if the reason for not upgrading is because you need to retain compatibility with some older sites, it would be possible to do this with IE9 by having those sites use the IE8 compatibility mode.
I normally recommend avoiding compatiblity mode, but there are times and places where it is useful, and I guess this may be one of them.
If you have a site which needs to use compatiblity mode, you can force it by using the X-UA-Compatible meta tag in your headers.
If you're dead set on sticking with IE8 as a browser, there is one option open to you to upgrade the rendering engine without changing your browser shell, and that's Google Chrome Frame.
Google Chrome Frame is a plug-in for IE6 and up, which allows the browser to use the Google Chrome rendering engine.
This will solve the problem for you in exactly the way you describe in the question, except that the engine you'd be upgrading to is Webkit rather than an updated version of Trident.
You can find out more about Google Chome Frame here: http://code.google.com/chrome/chromeframe/
Hope that helps.