Even Seriously Lotus doesn't claim an, "incredible improvement in handling," from their bushing kit. In fact the bushing they offer appear to be substantially OEM except for the two front upper A-arm rear bushings, which are marginally harder.
An "incredible improvement in handling" is hopping out of a delivery van into a Lotus. A very slight increase in hardness of two upper A-arm bushings isn't going to do it. They MIGHT be very slightly noticeable by a very experienced Lotus track driver.