I think any todays hard spring will satisfy you.
The Shepherd is most loved and also Xray has a new hard and constand spring.
I would advise you to take a look at the disc springs (conical washers) sold by Hudy, Xray, Parini and others. They do work great. The set mostly comes with 0.4mm and 0.6mm washers, get 2 sets and you can combine them from soft by using all 0.4mm washers to hard with only 0.6mm washers. all in between is made by a combination of 0.4 and 0.6mm washers.
Speed is only fiction but a 130+km/h MRS4 is for real.