You won't be disappointed with the I.M. Speedster. Its been a few years since I owned the original 58 Speedster but will try to do the comparison. It was a beautiful example with original unrusted floors & perfect original body. I really liked the way the original Speedster suspension felt, very compliant but very solid. It was equipped with koni's & as I recall the original suspension height. It had s 62 S90 engine with 1720 upper end & 44IDF Webers. It was fairly quick.
The I.M. has a stiffer ride even though now equipped with Koni's which I have set soft on the front & medium on the rear. To further improve the ride I have added Tecno Magnesium wheels with 185/60/15 Michelins. The main difference between the two suspensions is that the I.M. has an IRS rear suspension as opposed to the swing axle with the orig. Speedster. I have the I.M. set somewhat lower than my original so that may account for some of the stiffer ride. Having said that I still think the original suspension is more forgiving & is especially noticeable on an irregular surface.
The I.M. is notably faster with 145hp vs 90hp . The I.M. engine is the 2110 Type1 VW built by Pat Downs of CB Performance. The 2110 is a wonderful engine in every way , beautifully balanced with torque & useable HP from 2k rpm up thru 5500 rpm. It runs cooler than the S90 with much more usable HP & smoother overall performance. I have just around 34k miles on the engine so time will tell if it will give the same longevity as the original S90 . Pat Downs says with proper care it should go to 100k or more & that would compare nicely to the Porsche factory motors. Of course the 2110 has some advantages with it later technology , full flow oil system, external oil cooler , etc.
The gearboxs are wonderful in both examples, with the I.M.having a shorter shift gate & taller 4th gear for more relaxed highway cruising. The I.M. is notably noisier in 4th gear compared to the original Speedster gearbox.
Both example feel similar when behind the wheel, very solid & firm. The one piece body of the I.M. which is then bonded & riveted to the tubular chassis compares nicely to the steel unibody of the Original Speedster. The fit & finish on the I.M. is very impressive. The gaps on all closing panels are as good as those from Reutter & the doors & lids close with a sound & click as good as those produced in Stuttgart back in the day. Of course if you love the mystic of the original steel bodies , produced by wonderful craftsmen of Reutter along with the rich heritage of the Porsche Family & all the rich history of those connected to the Factory then there can be no comparison. But realistically many cannot afford the real Speedsters at their current prices so the replicas are a wonderful way to enjoy the pleasures of yesteryears, with the I.M. creating the perfect illusion of my Walter Mitty Fantasy.
In closing I would like to add I purchased my I.M. because its FUN TO DRIVE, ITS QUALITY BUILT, & ITS GORGEOUS & ITS AFFORDABLE.