It should do well over 70mph with no issues!
What are the tyres like? Try pumping them up further to something silly like 38 psi all round and see if that helps. OEM steering damper is good: heavy duty shouldn't be required. Is all the alignment good. Brake calipers and discs good?
Are you having a laugh? Did you read what he said, 70 MPH hitting a pothole on a B road. If you hit a pothole anywhere at 70 MPH you are not going to glide over it like Cinderella in her carriage are you. No matter how taut your suspension is.