Call their sales people and cancel your order if you like. I don't think you want to end up suffering from buyers remorse at the end of the day because of your doubts. Go with your gut, go to PTCL if required. That being said, last night at ~1am, I was downloading some Ad*be software from ParathaBay at 75 Kbps, so go figure.
As far as contention ratios go, I am not sure how ISPs implement them here, but if the ratios for PTCL are indeed 1:15, then each of those 15 odd users has a minimum reserved bandwidth equivalent to (total bandwidth / 15) or a maximum bandwidth of (total bandwidth). To put it in numbers, for a 512Kbps connection, the math works out to be:
Minimum Reserved Bandwidth (this absolutely the minimum speed you should ever experience, and might be the speed you experience during hours of high activity i.e. during the business day): 34kbps (4.2 KBps)
Maximum Bandwidth (this is the absolute maximum you can experience, as it assumes no one within your pool is using their connection, speeds you may experience during hours of low activity): 512kbps (64 KBps)
Now generally ISPs aren't completely stringent with their alloted bandwidth to the pool, i.e. a 1Mb pool may actually be alloted somewhere in the region of an extra 15-30% bandwidth to keep people happy. This would account for the occasionally insane speeds some people experience during the wee hours of the night or during wide power failures (I have experienced speeds in excess of 150 KBps for short bursts).
As a comparison, lets assume that the ISPs are actually stringent with their allotment (i.e. 512Kbps means 512Kbps) that would mean as a user of PTCL the lowest speed you could potentially experience is 4.2 KBps (based on a ratio of 1:15) and the lowest speed you could potentially experience as a Maxcom user would be 12.8 KBps (based on a ratio of 1:5).
I think for those who are looking for a *true* consistent, high speed experience they may want to seriously consider the ramifications of a high contention ratio.
Hope this helps.
p.s. I am NOT an employee or associated with Maxcom in any capacity. Just a community member.