There not restricted they just eventually run out of puff like most cars. Rumours were saying it went past 177 but GM were getting cold feet about the project so tamed it down a little. Thanks to 155 limit imposed by a lot of car companies the LC held on to the worlds fasted saloon for a bloody long time. It took Porsche over decade before they made a faster car. Any GM straight six 24v car will be in trouble over guides from now on. I had a set when I rebuilt the LC so I'm ok for another 100k miles. One guide recently sold for 500 quid on ebay, not bad for a 30 quid part. There has been rumours of a Finnish firm re marking them in Aluminium however I not sure how well that would work in a road engine. Problem with the standard part is it's plastic reinforced with a metal strip so not really down to making from just plastic.