not as bad as you've describe la bro. im using Singtel and my LTE and all wireless signal are acceptable when using the bumpers. at most sometimes lost 1-2 bars in crowded indoors like shopping centres.
I do agree that the e13ctron S4 series are hell of a good bumper but too bad they are now out of business already. so its impossible for us to look forward to their iphone 5S or 6 models.
I'm using Starhub, so that could also play a part. But in any case, I've noticed that the bars are not always an accurate reflection of the signal strength. Switching to a non-metal casing has really helped improve my reception (actual usage including surfing, etc) so I'm not very inclined towards going back to my metal bumpers even though I've spent quite a bit on them. Believe me, I really love my metal bumpers, they make the iPhone look really good.
Yes, it's a real pity that e13ctron are out of business already. I was really hoping to get something similar for my 5s.
try this: AL13 V2. 0 signal loss (as they claimed)
You poisoned me. I just went to their Kickstarter page while typing this reply and am now USD 60 poorer.

I really hope this will be the metal bumper I'm looking for.