I'm pretty well in the same boat.. I have almost enough (just some extra diff screws and bearings, and the motor mount) to build a new TA05, likely gonna sell it and make up some money from the upgrades, or just use it as a second-class car..
I certainly could have bought a 415MSX with the money I've spent, but I certainly wasn't at the skill level worthy of a carbon-aluminum chassis with LWS.. I had better things to worry about than tweak, balance, preparing CF and brittle suspension parts as a beginner.. heck, I'd have the CF deck split in two the in first week I'd try to race it..
The TA05 is an absolute TANK.. it's taken a LOT of abuse from my TCSing and foray into the indoor-foam 19T classes, and I've yet to break anything other than a 3 or 4 bearings and 2 melted diffs for overtightening them.. I'd be spending more time un-tweaking, repairing and buying parts than racing if I had anything else.. including my old TB-02 (two words.. crown gears..
Now I have the Xenon Chassis, bought second-hand but brand new, and it works like a charm, lighter and stiffer for indoors (now that TCS is done..). I'm still learning, but I can now drive well enough to avoid abusing the carbon chassis, a luxury I couldn't have been afforded before with my TB-02.
Now, in Xenon racing form, I'm at 1496g with IB3800's, a monster stock and foams.. still don't have the blue motor mount and the LW suspension, which I will consider for the outdoor TCS season.) If I switch to 19T and rubber, I'll likely be around the 1525g limit for rubber racing.. but then again, weight isn't everything.. another 70g to hit the foam limit is only 4.6% of the total weight.. even losing 100g from the stock weight of 1600 is only 6.3%, so in the grand scheme of things, comparing a TA05 to a 415MSX is still down to driving.. the best-of-the-best drivers may notice when it comes to the fastest of laptimes against 415's or T2's, but for the rest of us, it's more than enough for us to improve and race with..
Lotsa money went into my car, but I love it.. plus, most of the extra parts (suspension blocks, springs, screws, suspension, etc.. ) can easily be used in any future Tamiya car I purchase..
and I know I'll be well-prepped to handle it when it comes.