I would agree that the 960 straight six is inherently smoother, and a good V-12 even more so, based on having had both. However, I don't believe that the 5 cylinder NA is particularly noisy or heavy on vibrations. Are you feeling it once you get above idle speed? On my 850 NA and turbo, there is minimal vibration above that speed, even compared to our prior 960. I have replaced both torque bushing and "front" engine mount (on right side wheel) - torque bushing can go in as little as 30k, and seldom is ever known to go anywhere 100k. Front mount goes longer, but my first on the turbo was at 70k, and again at 101k due to buying an aftermarket one. Both are actually pretty easy to do, if you are handy. I have never replaced a hydraulic mount on either 850's, V70 or the 960. Note also, the 850's do transmit more noises than the960 or upgraded x70's due to less insulation, etc., just that vibration is not particularly an issue.
--
Mike Sullivan '07 XC70, '95 855T (100k), '95 855 NA (148k). Past Volvo's: '85 744 (256k), '86 245 (165k), '86 245 (195k), '88 745 (208k), '91 745 (196k), '93 965 (147k) , '98 V70XC (98k).