I love the Pista.
If I had a custom frame made, I'd tell the maker to use my Pista frame as the model.
My headset failed on me fairly early on, but then I weigh 235 when riding all up.
King makes the lightest and best headset, but also the most expensive.
Cane Creek has a whole range of headsets, from inexpensive to very expensive, and they all do a good job.
Soon after my headset failed, I broke a spoke, which happens not too often, but it happens.
Wheels, tubes and tires represent the most meaningful upgrade, in my opinion, but it costs money.
Cane Creek's Volos Track wheel set costs about $550, a slug of money, but they significantly reduce the rotational inertia of the wheels, which makes for better acceleration, braking (not everybody skids), and climbing (unless the rider has an absolutely perfect spin, he or she must re-accelerate his wheels against gravity twice per rpm).
A Continental Four Season tire in front, Continental Gatorskin in back, and Salsa Superlite tubes, with the Cane Creek wheels, costs close to $650 but it makes for a very light, strong, grippy, flat-resistant and quick package.
A Stronglight 130 BCD crank from John Dace at Businesscycles for less than $125 provides the lightest crank available, with plenty of strength and precision.
Combine this with a Phil 108mm JIS bottom bracket for $109, and Phil cups for $35 and TWO bottom bracket tools for $28.
This gives access to the world of cheap, light and efficient 130 BCD chainrings.
I have 39t, 42t, 46t, 47t, 52t and 53t chainrings.
Use only EAI cogs.
Do not switch cog manufacturers because the slight diffences in threading will shorten the life of the rear hub.
I have 16t, 17t, 18t and 19t EAI cogs.
A 17t and 19t cog, with 39t, 42t, and 46/47t chainrings, provides a huge range of gear inches, and allows a lot of experimentation.
I have ridden 82 gear inches to 54 gear inches, and everything in between.
They all have merit depending on the situation.
I have found hard saddles more comfortable, in the long run, than soft saddles.
Selle San Marco makes a nice Ti Aero Aspide that weighs less than 200g and costs less than $125.
Jenson and Performance both list this saddle.
Not everyone likes it, but a lot of people do.
I like bullhorns and bar end brake levers.
They don't cost much.
I invested in a bunch of very cheap stems of various lengths and degrees of rise.
I went through seven or eight combinations before I found out that 90mm and 6 degrees of rise works for me on the street.
Then I bought a higher quality (and more expensive) stem of those dimensions.
I ride with Shimano 970 double-sided SPD pedals, but 540's do just as well for half the money.
These pedals make sure I can clip in effortlessly in a hurry, and in three years I have unclipped twice, both times just a little earlier than I planned.
I brake by back pedaling (I ride on expensive tires and I get a year out of them), and I stop quickly, and I have never unclipped while braking or at high spin rates.
Shimano's cheapest Mountain Bike shoe does a great job and I can walk in them very comfortably.
I sometimes forget I have them on my feet.
So, Cayman, upgrade as you can afford it.
I makes for a great hobby, and I enjoy the planning, saving, and anticipation almost as much as putting the upgraded part on the bike.
I've shared what I did with my Pista, and you will probably make different choices.
Have fun.