Yes they allow you to travel a little bit faster in a non-competitive environment whilst wearing out a lot quicker and costing a lot more in the first place.
For the £7000 you suggest he spends on a £400 road car with no roll cage or shell preparation you could easily go and buy a single seater with spares and a trailer to tow it with. Or frankly you could go and by any well sorted racing hot hatch/saloon and blow and badly built over weight road car away on both pace and performance.
Touching the engine should be the last thing on the list. Anything other than a cheap set of dampers and anti-roll bars is going to be lost on a car without a rollcage and the idea of spending £2000 on a set of wheels and road tyres is ridiculous. For £7000 you could buy and prepare a decent Silvia, buying the cheapest car you can find is a false economy, putting a full cage and doing neccessary shell work is the starting point if you're going to build a good car.
...and you prove how clueless you are, the kerb weight for a mk. 1 MX5 is 940kg, you'll struggle to get a fully stripped Silvia to get anywhere close to the MX5 with every bit of trim and equipment it left the factory with.