I was one of the first people here running the "rods only" build, look it up
On e85 it's possible, on pump gas it's a roll of the dice. The difference between 250 and 350 is as Trump would say: UUUGE
Sounds like you should just keep learning and saving your pennies, because being clueless and broke is a really bad position to be in when building/turbocharging an n/a car.

Miata's are really cheap to build. Miata's are really cheap and easy to turbo. If a set of $400 pistons and $300 in machine work is enough to break your budget, you're not ready to do any of this.

 

Oh but I didn't say it would be enough to "break my budget" as you call it, but why buy something overkill if what you already have works fine (if the stock pistons would be enough). Another engine would be sort of a side project I'll be working on while the car is in running condition, there's no need to rush it so I will be able to spend the money necessary to get it to where I want it. Also, despite the lack of E85 stations here, I intend to upgrade to "UUUGE" injectors and a matching fuel pump, along with a flexfuel sensor and new lines to run E85. I just graduated university and there's lots of jobs currently available, so I won't have to worry about "the budget" all that much

I know Miata parts are relatively cheap, but I thought that the only way to get forged pistons was to get them from a US store, at which point the high-$400 pistons turn into ~$700 pistons. Luckily, they're for sale in the UK for something like $630 converted from euros. But again, if it wasn't necessary I would've skipped the pistons, but it seems like they are necessary, especially since my main intention is to have a reliable car.

 

if you don't have a budget to break, the start the correct way:

buy the proper ms (3 is the minimum for a vvt engine swap), install it and learn everything about it and how it controls your car and how to tune it all.
then, about a year later at the soonest, you'll not only have more knowledge and experience with this, but also more money to spend, and by that time you'll probably have more parts available too. win/win

if you're planning to go over 300 on pump gas, pistons are basically mandatory

 

And Wiseco's are stronger pistons that Supertech. They require a larger piston/wall clearance ( where the piston slap stories come from ) but they are the strongest alloy for a forged piston and can take the abuse. My machinist went with a .035 PW clearance and its been perfect, no slap and very minimal oil burning. My last set of Wisecos had 65k on them and still could have been reused again. I went with a larger bore this time but the fact that 65k hard boosted miles and the pistons looked perfect is the reason I use Wiseco.