I'm actually going to disagree with some posters and say a better bike (stiffer, lighter) will probably make you faster (though I doubt it would be as much as 2 MPH). It sounds like you've put in the hard work fitness wise so you know 2 things
1) You like riding, and you're going to be doing a good deal more
2) As far as fitness goes, you're now at the point of diminishing returns. It's like you've knocked down the low lying fruit (fitness, bike handling, group riding etc) so I see a new bike as totally justifiable if you want it
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