Changing back to 4 speed

Extremely strange about the slower acceleration.
The minimal extra weight of the Type 9 definitely wouldn't have a noticeable affect.
Have you checked each of the gear ratios for each gear and compared with the original gearbox gear ratios?
 

I read that the 4 speedbx is 22kg and that the type 9 weighs 11 kgs more:
 
http://www.type9gearbox.co.uk/type-9-history.html
 
 
 

You're running a 1600 aren't you I think ?  If so I'm not sure you're going to like the 3.54 too much.  I'm a real fan of the Kent motors and have them in both of my Escorts (thats a 1600 in my profile pic) with no plans to change.  But they're no torque monster.  I wouldn't go near a 3.54 without a 2 litre up front.  
 
As to the first/second gap not sure there is much you can do short of putting in a Rocket box but then you're initial take off will be slower, once moving they are great and that gap disappears, first gear becomes useful again.
 
I really would have thought the 4.1 and Sierra box would have been an awesome combination.  I didn't realise the weight difference between the 4 and 5 speed was so much.  Bear in mind that the extra 11 kg's isn't all going to be in rotating parts, some is going to be the case, etc.

You can get them with a taller first gear in the UK:
 
http://www.bghgeartech.co.uk/html/4_speed.html
 
Just don't look at the price!

Yes, all the same ratios as a 4 speed. Slower acceleration as in 0.3 second over 400m

I am assuming you have actually checked (ie stuck your head in there) the T9 gear ratios in YOUR gearbox, not just reading technical documentation which is often of a general nature.
 
With the change to the 4.125 diff ratio the super short standard Escort first gear will run out of puff much faster than with the 3.77 and especially the 3.54 ratios. Same with the second gear. If your engine is producing 130HP, torque will also be increased. So the engine wants to work because of the extra torque, but can't apply the torque correctly with the shorter diff ratio. So the solution is to change the gear ratios, either in the gearbox, or the diff, or even maybe both. A change in wheel diameter can also help a lot.
 
I turbocharged my RS2000 in 1985 and retained the 4 speed gearbox and the 13 inch wheels. Acceleration and cruising effort and the entire characteristics of the car changed when I fitted a 2.72:1 diff. The car went from being just "fast" to "a weapon" that none of my friends with modified V8's could touch. The secret was in the gearing.
 
Three years ago I changed the diff ratio of my stock standard 1979 1600 Escort panel van from 3.77 to 3.54. The car is my daily driver. Even with good loads and a full tank of fuel going up hills there is more than enough power. The benefit is longer legs, reduced in car noise levels and much improved fuel economy. I am a fan of longer legs on street cars.

If you want to use the car cruising at higher speeds often you will probably like the 3.54 however i much rathered the 3.77. I run a 2L with around the same power (probably a touch less) and 205/60r13 rear tyres. Also standard 4 speed.
 
I dont do much long distance driving at higher speeds so I dont mind doing extra revs. Also I do a bit of track work and hillclimbs where I want the extra revs at a given speed.
 
Car is much more lively with 3.77 than 3.54. It is noticably faster. Will be instaling a 3.9 with lsd (not sure if 3.9 is accurate but there-abouts) soon to give better track performance and no doubt will be screaming when cruising at 100km/h.
 
A shorter diff ratio will no doubt compromise economy and high speed cruising comfort. Even though my car is daily driven I feel its much more important for it to be more responsive on the street and quicker at the track than save a couple extra $$$ when I fill up.
 
Ultimately it depends what you want the car to do
 
Cheers,
Ryan