Then yeah, the manifold sucks.
Did you figure out which engine you have with surety? Just to keep your spending in check: Ford makes a racing manifold for the Zetec that is fairly inexpensive from FordRacing. IIRC (if I recall correctly) it's like $200. Used versions might be available on this site from time to time much cheaper than that. FordRacing makes no Dtec intake that I know of, but Cosworth and a Chinese rough finish copy are available for $700 or $400 respectively. I think there is another one available from Top Speed, one of our other vendors, for somewhere between those 2 prices IIRC.
If you were making a Dtec intake in the layman's scope, ie: without flow bench, dyno, etc, the first thing I'd do (besides buy a TIG) is use a fabric measuring tape to measure the stock runner length and rough sq" of the port. That would give a decent starting point. From then on what applies to V8 manifolds as far as balancing velocity and volume to provide maximum performance in X rpm would still apply to EFI manifolds.
1TurboFocus has a flow bench, and has done some experimenting with manifolds on Dtecs. IIRC, what he said is that the factory manifold flows great for street performance as is. It's hard to beat IAT's of plastic manifolds even with aluminum, so that's one point. The HP Dtec manifolds like the Cosworth are shorter runners tuned for mid-high rpm usage, and I can refer you to threads of woes and dynos proving there's a drop in torque/increase in max torque rpm like you'd expect from increased volume and lower velocity.
Given the layman's info I mentioned at first, I'd try to balance out increases in volume with increases in runner length to equalize velocity. It's a hobby, right? All you're really wasting is time and a little money for fun. It's bound to be more exciting and educational in the long term than visiting a casino. Especially if you're like me and seem to walk away from every machine just before it hits.
OH, for a real money pit, research how much it costs to build an electric hot-rod or drag racer with an AC motor- the only way to do regenerative braking easily, and have rpms better than diesel. DC motors are good for drag racing- but not for cruising. AC motors are like 3x as expensive. It is possible to spend about half what a Volt retails for and get something with 100 mile battery. Now if sizable 400 hz generators and electronics were available then more suitable range extending engines could be found and utilized for about the same cost as a Volt. That would give the DIYer a plug-in hybrid with 100 mile charge range unlimited speed of all elec driving, and similar hybrid performance even without the engine and range extending generator being connected to the transmission via clutches. I digress..