Bike Upgrade Question

I have been in the sport for 3-1/2 years.  I do the 70.3 distance races and have improved from 18.5 MPH average to 22 MPH average.  Here is what I did.

1) First bike was a 17 year old Aluminum Frame TT bike with stock 650C stock wheels.  I did my fist race in Speedo Jammer Swim Shorts and an Under Armor compression shirt.  I averaged 18.5 MPH for 56 miles after 4 months of training.

2) My bike mechanic told me that two things were worth spending money on. First a good seat.  Second a set of race wheels.  He said that any time you up grade your bike you just switch them out and put them on the new bike.  He also told me that the gearing on the bikes with 700C wheels would work out better for me.  My $300 bike had a high end bike seat on it  so I kept the seat and my first upgrade was to a 9 year old Carbon Frame bike with 700C stock wheels.  

3) My next purchases were to upgrade the Speed Swim shorts and compression shirt to a Tri Suit, Latex tubes for my continental Grand Prix Tires, and an XLab between the aerobars aero hydration system with a straw, . 

4) Next was the Aero Helmet. 

5) After all that I went for the HED carbon Rear Disk Wheel and Tri Spoke Front wheel.

6) About all I have done since then was to change out the platform petals with toe clips for speed play pedals and LG bike shoes.

I didn't see an overnight speed increase with any single upgrade. When I put on these new tires, that new helmet or the new wheel set I found my speed was nearly identical to what I was doing the date before on the other equipment. I think they gave me just enough an edge every time to keep moving forward and to not plateau.  

Bite the bullet...

This article linked below is 6 years old now but offers a good comparison for a RB vs TB and still applies. Along with all the aero support goodies (wheels, helmet, water bottle, storage, etc.), don't forget about using proper tires with good Crr and latex tubes.

https://www.bikeradar.com/us/road/gear/article/how-aero-is-aero-19273/